message: build: Fix Boost.Process detection on macOS arm64
  Could be tested as follows:
  ```
  % brew install
  % ./autogen.sh
  % ./configure --with-boost='/opt/homebrew/opt/boost@1.76'
  ```
  Github-Pull: bitcoin#24521
  Rebased-From:
diff: @@ -1446,6 +1446,8 @@ 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 $PTHREAD_CFLAGS"
  @@ -1453,6 +1455,7 @@ if test "$use_external_signer" != "no"; then
  [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"
  