Merge bitcoin#24521: build: Fix Boost.Process detection on macOS arm64

1d4157a build: Fix Boost.Process detection on macOS arm64 (Hennadii Stepanov)

Pull request description:

  Could be tested as follows:
  ```
  % brew install [email protected]
  % ./autogen.sh
  % ./configure --with-boost='/opt/homebrew/opt/[email protected]'
  ```

ACKs for top commit:
  promag:
    Tested ACK on 1d4157a with boost 1.76 on macOS arm64. bitcoin#24523 is required for boost 1.78.

Tree-SHA512: 7abd39a78e970ecc051e53b5923dfc31d3f0576cf4ff7fcfb9c8708c857c46a7a595ec36238def83f41158970eeee209980da4b8b70f0ff68f940a38ac9a0471

Author: fanquake

configure.ac

@@ -1446,13 +1446,16 @@ if test "$use_external_signer" != "no"; then
     ;;
     *)
       AC_MSG_CHECKING([whether Boost.Process can be used])
+      TEMP_CPPFLAGS="$CPPFLAGS"
+      CPPFLAGS="$CPPFLAGS $BOOST_CPPFLAGS"
       TEMP_LDFLAGS="$LDFLAGS"
       dnl Boost 1.73 and older require the following workaround.
       LDFLAGS="$LDFLAGS $PTHREAD_CFLAGS"
       AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <boost/process.hpp>]])],
         [have_boost_process="yes"],
         [have_boost_process="no"])
       LDFLAGS="$TEMP_LDFLAGS"
+      CPPFLAGS="$TEMP_CPPFLAGS"
       AC_MSG_RESULT([$have_boost_process])
       if test "$have_boost_process" = "yes"; then
         use_external_signer="yes"