Autotools: Include only php_config.h in libmbfl

This is a sync with the include style used as of PHP-8.4 to be able to
do simultaneous in-source and out-of-source builds.

When building with phpize there isn't any symbol used from the generated
extensions's config.h but the php_config.h is needed for some
preprocessor macros to be resolved properly.

Author: petk

ext/mbstring/config.m4

@@ -163,14 +163,8 @@ if test "$PHP_MBSTRING" != "no"; then
     PHP_ADD_INCLUDE([$ext_builddir/$dir])
   done
 
-  out="php_config.h"
-
-  if test "$ext_shared" != "no" && test -f "$ext_builddir/config.h.in"; then
-    out="$abs_builddir/config.h"
-  fi
-
   cat > $ext_builddir/libmbfl/config.h <<EOF
-#include "$out"
+#include <main/php_config.h>
 EOF
 
   PHP_INSTALL_HEADERS([ext/mbstring], [mbstring.h])