$ext_src_base is the source dir of the extension

acinclude.m4

@@ -350,6 +350,7 @@ dnl a dynamically loadable library.
 dnl
 AC_DEFUN(PHP_EXTENSION,[
   if test -d "$cwd/$srcdir/ext/$1" ; then
+    ext_src_base="${abs_srcdir}/ext/$1/"
     ext_base="ext/$1/"
     EXT_SUBDIRS="$EXT_SUBDIRS $1"
     if test "$2" != "shared" && test "$2" != "yes" && test -z "$php_always_shared"; then