Merge bitcoin/bitcoin#31013: depends: For mingw cross compile use -gcc-posix to prevent library conflict

ae56b3230b depends: For mingw cross compile use -gcc-posix to prevent library conflict (laanwj)

Pull request description:

  CMake parses some paths from the spec of the C compiler, assuming it will be the linker, resulting in the link to end up with  `-L/usr/lib/gcc/x86_64-w64-mingw32/12-win32` on debian bookworm if both `-win32` and `-posix` variants are installed, and `-win32` is the default alternative.

  This results in the wrong C++ library being linked, missing std::threads::hardware_concurrency and other threading functions.

  To fix this, use the `-posix` variant of gcc as well when available. This fixes a regression compared to autotools, where this scenario worked.

ACKs for top commit:
  theuni:
    utACK ae56b3230b.
  hebasto:
    ACK ae56b3230b. I've tested on both Debian Bookworm and Ubuntu 24.04 with the `g++-mingw-w64-x86-64` package installed. The resulting CMake internal configuration appears more accurate. For instance, on Ubuntu 24.04, for the `bitcoin-tx` target, the diff in `build/src/CMakeFiles/bitcoin-tx.dir/linkLibs.rsp` looks as follows:

Tree-SHA512: f36fae50f91a29f565940494af9e46f47e219b99e329c0714ace47c516ac524602d5b6538a07488157bc2a71be7bac72176097fff3178129c5084bf6cc823167

depends/hosts/mingw32.mk

@@ -1,3 +1,6 @@
+ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-gcc-posix"),)
+mingw32_CC := $(host)-gcc-posix
+endif
 ifneq ($(shell $(SHELL) $(.SHELLFLAGS) "command -v $(host)-g++-posix"),)
 mingw32_CXX := $(host)-g++-posix
 endif