Merge #18558: build: Fix boost detection for arch armv7l

da0842dcd4 build: Update ax_boost_mase.m4 to the latest serial (Hennadii Stepanov)

Pull request description:

  Picked from the upstream 90814f1895

  Fix #17010.

  This PR is [alternative](https://github.com/bitcoin/bitcoin/issues/17010#issuecomment-610651736) to #18501.

ACKs for top commit:
  laanwj:
    ACK da0842dcd4

Tree-SHA512: 5e43e12c524e4ea6b967c9be02c81a75948eac6cf55b819e3339222a2e3414731581d40af3524ad865abae7c5247c190448ebf2aa5e0d9a338edb501cc23ba38

build-aux/m4/ax_boost_base.m4

@@ -33,7 +33,7 @@
 #   and this notice are preserved. This file is offered as-is, without any
 #   warranty.
 
-#serial 47
+#serial 48
 
 # example boost program (need to pass version)
 m4_define([_AX_BOOST_BASE_PROGRAM],
@@ -123,6 +123,7 @@ AC_DEFUN([_AX_BOOST_BASE_RUNDETECT],[
     dnl are almost assuredly the ones desired.
     AS_CASE([${host_cpu}],
       [i?86],[multiarch_libsubdir="lib/i386-${host_os}"],
+      [armv7l],[multiarch_libsubdir="lib/arm-${host_os}"],
       [multiarch_libsubdir="lib/${host_cpu}-${host_os}"]
     )