Fix build issues on macOS (bug#29931)

* configure.ac: On darwin add check for Homebrew texinfo install, and
fix incorrect quoting of libxml2 includes.

configure.ac

@@ -1267,6 +1267,14 @@ esac
 AC_SUBST([PAXCTL_dumped])
 AC_SUBST([PAXCTL_notdumped])
 
+# Makeinfo on macOS is ancient, check whether there is a more recent
+# version installed by Homebrew.
+AC_CHECK_PROG(HAVE_BREW, [brew], [yes])
+if test -n "$HAVE_BREW"; then
+  AC_PATH_PROG([MAKEINFO], [makeinfo], [],
+    [`brew --prefix texinfo 2>/dev/null`/bin$PATH_SEPARATOR$PATH])
+fi
+
 ## Require makeinfo >= 4.13 (last of the 4.x series) to build the manuals.
 if test "${MAKEINFO:=makeinfo}" != "no"; then
   case `($MAKEINFO --version) 2>/dev/null` in
@@ -3881,13 +3889,13 @@ if test "${with_xml2}" != "no"; then
 	xcsdkdir="" ;;
       esac
     fi
-    CPPFLAGS="$CPPFLAGS -I$xcsdkdir/usr/include/libxml2"
+    CPPFLAGS="$CPPFLAGS -isystem${xcsdkdir}/usr/include/libxml2"
     AC_CHECK_HEADER(libxml/HTMLparser.h,
       [AC_CHECK_DECL(HTML_PARSE_RECOVER, HAVE_LIBXML2=yes, ,
 		     [#include <libxml/HTMLparser.h>])])
     CPPFLAGS="$SAVE_CPPFLAGS"
     if test "${HAVE_LIBXML2}" = "yes"; then
-      LIBXML2_CFLAGS="-I'$xcsdkdir/usr/include/libxml2'"
+      LIBXML2_CFLAGS="-isystem${xcsdkdir}/usr/include/libxml2"
       LIBXML2_LIBS="-lxml2"
     fi
   fi