Merge bitcoin/bitcoin#21231: Add /opt/homebrew to path to look for boost libraries

9a09695 build: Add /opt/homebrew to path to look for boost libraries (Fu Yong Quah)

Pull request description:

  Following the instruction in https://github.com/bitcoin/bitcoin/blob/master/doc/build-osx.md for building on the M1 Macs don't work out of the box, because homebrew now defaults to /opt/homebrew instead of /usr/local. This PR fixes that.

ACKs for top commit:
  jonasschnelli:
    utACK 9a09695
  promag:
    Tested ACK 9a09695.

Tree-SHA512: 472568b97fbd8623481fe6fd43b0509fa32fe7f1c1d8090321a6a6a5bdc7343d4ad4122c10dcc7c9c93068db8a3f009a73befaf1ba11e4af54a66afd2c2dbe14

Author: laanwj

build-aux/m4/ax_boost_base.m4

@@ -11,7 +11,7 @@
 #   Test for the Boost C++ libraries of a particular version (or newer)
 #
 #   If no path to the installed boost library is given the macro searchs
-#   under /usr, /usr/local, /opt and /opt/local and evaluates the
+#   under /usr, /usr/local, /opt, /opt/local and /opt/homebrew and evaluates the
 #   $BOOST_ROOT environment variable. Further documentation is available at
 #   <http://randspringer.de/boost/index.html>.
 #
@@ -151,7 +151,7 @@ AC_DEFUN([_AX_BOOST_BASE_RUNDETECT],[
         else
             search_libsubdirs="$multiarch_libsubdir $libsubdirs"
         fi
-        for _AX_BOOST_BASE_boost_path_tmp in /usr /usr/local /opt /opt/local ; do
+        for _AX_BOOST_BASE_boost_path_tmp in /usr /usr/local /opt /opt/local /opt/homebrew/; do
             if test -d "$_AX_BOOST_BASE_boost_path_tmp/include/boost" && test -r "$_AX_BOOST_BASE_boost_path_tmp/include/boost" ; then
                 for libsubdir in $search_libsubdirs ; do
                     if ls "$_AX_BOOST_BASE_boost_path_tmp/$libsubdir/libboost_"* >/dev/null 2>&1 ; then break; fi
@@ -227,7 +227,7 @@ AC_DEFUN([_AX_BOOST_BASE_RUNDETECT],[
             fi
         else
             if test "x$cross_compiling" != "xyes" ; then
-                for _AX_BOOST_BASE_boost_path in /usr /usr/local /opt /opt/local ; do
+                for _AX_BOOST_BASE_boost_path in /usr /usr/local /opt /opt/local /opt/homebrew ; do
                     if test -d "$_AX_BOOST_BASE_boost_path" && test -r "$_AX_BOOST_BASE_boost_path" ; then
                         for i in `ls -d $_AX_BOOST_BASE_boost_path/include/boost-* 2>/dev/null`; do
                             _version_tmp=`echo $i | sed "s#$_AX_BOOST_BASE_boost_path##" | sed 's/\/include\/boost-//' | sed 's/_/./'`