Merge #21403: build: set --build when configuring packages in depends

735610940c build: set --build when configuring packages in depends (fanquake)

Pull request description:

  After reading [#Specifying-Target-Triplets](https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/autoconf.html#Specifying-Target-Triplets) in the autoconf manuel, my understanding is that this change should mostly be a no-op, as `--build` defaults to the output of [`config.guess`](https://github.com/bitcoin/bitcoin/blob/master/depends/config.guess), however, this may be slightly more correct

  >  For historical reasons, whenever you specify --host, be sure to specify --build too; this will be fixed in the future.

  and will quell some warnings in depends (#16354), i.e:
  > configure: WARNING: If you wanted to set the --build type, don't use --host.
      If a cross compiler is detected then cross compile mode will be used.

  If anything, this also explicitly enables cross-compilation mode when `--host` differs from `--build`. As for "fixed in the future", this is the case for Autoconf 2.70+.

  If we don't make this change, I think we should consider closing #16354. We've now got a good handle on our depends configure flags, and we can just leave the last few warnings as they are. It's also unlikely that we are going to add the `xorg-macros` package to depends.

  Guix builds at 735610940c0cac72343ca247eb3062b2e09512a0:
  ```bash
  bash-5.1# find output -type f -name *$(git rev-parse --short HEAD)*.* -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
  afabf99650d57abd3df2c7e6253a3281f9814ada77d01c28ffb57c740b14fd8e  output/bitcoin-735610940c0c-aarch64-linux-gnu-debug.tar.gz
  6e9dedced61489f7734c5387ee05823db68eb4b890fa05f11541c1b966576446  output/bitcoin-735610940c0c-aarch64-linux-gnu.tar.gz
  5333a3c5978a61966251e56fedab8e059d9d8a1a7231a0908095bb373204a81f  output/bitcoin-735610940c0c-arm-linux-gnueabihf-debug.tar.gz
  d28e1c166b96e58cf40b3f9801c3050c2785b25eeb261d44eeb31a32a75e134c  output/bitcoin-735610940c0c-arm-linux-gnueabihf.tar.gz
  e00b0149c52e9207be1cfb56ec3b715b55f41691401095f7a7038ac124f08d80  output/bitcoin-735610940c0c-osx-unsigned.dmg
  ae5052ad1a05916bc6656b034c169097471fd9bdba16b606e2bcb592c9395544  output/bitcoin-735610940c0c-osx-unsigned.tar.gz
  8586a9b8b82a8832397fcd0756aa9649adc3e61ae579fb370b99fe406298199f  output/bitcoin-735610940c0c-osx64.tar.gz
  d560a8781bae856b6593e9b731fa99de0a4ad7bd3e03084166dc98904ece9e7b  output/bitcoin-735610940c0c-powerpc64-linux-gnu-debug.tar.gz
  a24cf28b7efd5ebdc892e7e8809a05b3405bb4222f7d2eaa32de265214c78f28  output/bitcoin-735610940c0c-powerpc64-linux-gnu.tar.gz
  ad29421842dbbe96526eb597a7ff2970f0acae8c3e75dc6c2100716a2ba20606  output/bitcoin-735610940c0c-powerpc64le-linux-gnu-debug.tar.gz
  ee7a8ccfd68cc2297bb96eaaacf7b3eb939a5e39cecc3c710dbe828518f1bfbd  output/bitcoin-735610940c0c-powerpc64le-linux-gnu.tar.gz
  185bf4ea3b35072a8dd54aedbafc0892ec1e486c8ebd311f05f0fb47a19058cf  output/bitcoin-735610940c0c-riscv64-linux-gnu-debug.tar.gz
  803078b15a9bbaa567176827639895e762edbe80844b80400f8a9581e5311d32  output/bitcoin-735610940c0c-riscv64-linux-gnu.tar.gz
  d920327a32a42a5abadd247f0d3f3ee0ab0488a28892c11838d99bede6456723  output/bitcoin-735610940c0c-win-unsigned.tar.gz
  9f1741a37e294af153d57ad4a6dad3e5f1cdf75f1588c86cb467abf0c177ae27  output/bitcoin-735610940c0c-win64-debug.zip
  2d1a91552c0dd7bb400ce6c71d2e98de702743a97afe5f4b0945865f251f3aea  output/bitcoin-735610940c0c-win64-setup-unsigned.exe
  8a70020bb0cb68516acf4b2cc89832d72373f31611bd075826a30a817a071cd2  output/bitcoin-735610940c0c-win64.zip
  93b03837679474905538e96c44ba81ee5653795ad333dc00569b92c82dc6ea31  output/bitcoin-735610940c0c-x86_64-linux-gnu-debug.tar.gz
  6ccd046a83ef5089667ad426875319beb83e2b2a5fb4d4459d5c5ce8d75f1a31  output/bitcoin-735610940c0c-x86_64-linux-gnu.tar.gz
  12771acb17de84cda8e81c59704c00b1d4725ff51fa84c9067dc61a8dc795bdc  output/src/bitcoin-735610940c0c.tar.gz
  ```

  Gitian builds:
  ```bash
  # macOS:
  7d087749f610e5e3eca91c730d41afd4d404808e276242d05cedb89403de247f  bitcoin-735610940c0c-osx-unsigned.dmg
  96d68b4f0042dc40f5e8e362fa84dcf8e122dcb566889feb48ad8a982847dd8b  bitcoin-735610940c0c-osx-unsigned.tar.gz
  2dea72cbe5c3e228babd88b516a6f43b0e040e9510ea95b62f07fc0815bf8d22  bitcoin-735610940c0c-osx64.tar.gz
  12771acb17de84cda8e81c59704c00b1d4725ff51fa84c9067dc61a8dc795bdc  src/bitcoin-735610940c0c.tar.gz
  478dbda9d551a8bed985c61c625df3f543afab26989a52bc44d5a4745715af81  bitcoin-core-osx-22-res.yml

  # Windows:
  c31a5118dfa21259b9868e2c4858c635becb786c2ee8af92302da7811a1efe9b  bitcoin-735610940c0c-win-unsigned.tar.gz
  5c171ec3aee830abc8ae843d674d5fe7a57420e782ee765b6992914ae5b7671a  bitcoin-735610940c0c-win64-debug.zip
  ca010ced982f75d111e01f3759b961deb83e7bf511b1ee9c9041f4830570f2a0  bitcoin-735610940c0c-win64-setup-unsigned.exe
  62514b01281f81af6057fb12cc5e3813c1d8f6532d0d8ac8f6f2909a595aa1dd  bitcoin-735610940c0c-win64.zip
  12771acb17de84cda8e81c59704c00b1d4725ff51fa84c9067dc61a8dc795bdc  src/bitcoin-735610940c0c.tar.gz
  e26b55d8302a419f594396db6ff441aaeb5e275e02fde7b1b4eb092270f902e7  bitcoin-core-win-22-res.yml

  # Linux:
  e5be4deb91cb372de484468c0575a04f907f982e35a9841659bb213540f770d0  bitcoin-735610940c0c-aarch64-linux-gnu-debug.tar.gz
  6169e822679ece2bb6af7027362a6f373eb939eef9d89edc58c21190b4083c7d  bitcoin-735610940c0c-aarch64-linux-gnu.tar.gz
  aee5308767c5c930853a80ab94325b2a4ab3dbe9a7a20c7a9d60d57af2b5383f  bitcoin-735610940c0c-arm-linux-gnueabihf-debug.tar.gz
  a9cf6e56d859fffd5a2b5f1e6360515bc6b06b048776b93cc08954859457a251  bitcoin-735610940c0c-arm-linux-gnueabihf.tar.gz
  7617d85c358e6614fd40026a8d346f9edba5b5139e3daade2ec2aec72aed3a1f  bitcoin-735610940c0c-powerpc64-linux-gnu-debug.tar.gz
  f76e46a8b130812541ebcaaba173b4cbe2ec1781e348579b44f96e9244cc2475  bitcoin-735610940c0c-powerpc64-linux-gnu.tar.gz
  19ada7abb801422bd7772d7ee559004cab11a46141a747aabd27085e8c3d200d  bitcoin-735610940c0c-powerpc64le-linux-gnu-debug.tar.gz
  f3a6bbefe9f47b3960e3b9eeb3f740ecddf8773ae76961ced6a3cdf82503e37f  bitcoin-735610940c0c-powerpc64le-linux-gnu.tar.gz
  42ca333e8f82b4ec0b2354efc7c4f1c27ed32d60c35b88e33d26833419f19e69  bitcoin-735610940c0c-riscv64-linux-gnu-debug.tar.gz
  dde930d9134fb5be1374014db3f6f503b614ae801a1d77f6f4b82a5f11eb2518  bitcoin-735610940c0c-riscv64-linux-gnu.tar.gz
  82e1b4cc8aac309a99458cc27fd0285215a8af53780094c4990303ce8948cf0c  bitcoin-735610940c0c-x86_64-linux-gnu-debug.tar.gz
  ee8caad2ccddd8eaf8aa2d19040d859130168183355b09a2a0f77a59a97fcaee  bitcoin-735610940c0c-x86_64-linux-gnu.tar.gz
  12771acb17de84cda8e81c59704c00b1d4725ff51fa84c9067dc61a8dc795bdc  src/bitcoin-735610940c0c.tar.gz
  d90545b13226a338c994d9e538f468a6fafd694273bf71e14614feeaf5727ad4  bitcoin-core-linux-22-res.yml
  ```

ACKs for top commit:
  dongcarl:
    Code Review ACK 735610940c
  hebasto:
    ACK 735610940c, tested on Linux Mint 20.1 (x86_64).

Tree-SHA512: 13ce05269309a12d03b81a539ba5585003785f7b1bb7894a9353385a6e9c709f920cc4f79322f28d7a256809754f201e70be6b917915c7b14e1461530075781d

depends/funcs.mk

@@ -140,7 +140,13 @@ $(1)_config_env+=CMAKE_MODULE_PATH=$($($(1)_type)_prefix)/lib/cmake
 $(1)_config_env+=PATH=$(build_prefix)/bin:$(PATH)
 $(1)_build_env+=PATH=$(build_prefix)/bin:$(PATH)
 $(1)_stage_env+=PATH=$(build_prefix)/bin:$(PATH)
-$(1)_autoconf=./configure --host=$($($(1)_type)_host) --prefix=$($($(1)_type)_prefix) $$($(1)_config_opts) CC="$$($(1)_cc)" CXX="$$($(1)_cxx)"
+
+# Setting a --build type that differs from --host will explicitly enable
+# cross-compilation mode. Note that --build defaults to the output of
+# config.guess, which is what we set it too here. This also quells autoconf
+# warnings, "If you wanted to set the --build type, don't use --host.",
+# when using versions older than 2.70.
+$(1)_autoconf=./configure --build=$(BUILD) --host=$($($(1)_type)_host) --prefix=$($($(1)_type)_prefix) $$($(1)_config_opts) CC="$$($(1)_cc)" CXX="$$($(1)_cxx)"
 ifneq ($($(1)_nm),)
 $(1)_autoconf += NM="$$($(1)_nm)"
 endif