improve temporary fix for system libgd

- PHP_GD_PNG needed to set include path for png.h
- PHP_GD_JPEG needed to set include path for jpeglib.h

Author: remicollet

ext/gd/config.m4

@@ -177,7 +177,13 @@ dnl Various checks for GD features
     ], [ $GD_SHARED_LIBADD ], [char foobar () {}])
 
   else
+    dnl for png.h in gd_compat (PNG_LIBPNG_VER_STRING)
+    PHP_GD_PNG
+    dnl for jpeglib.h in gd_compat (JPEG_LIB_VERSION)
+    PHP_GD_JPEG
+    dnl for X11/xpm.h in gd.c (XpmLibraryVersion)
     PHP_GD_XPM
+    dnl for ft2build.h in gd.c (FREETYPE_MAJOR...)
     PHP_GD_FREETYPE2
 
     extra_sources="gd_compat.c"