Fail early when building ext/gd without ext/zlib

The bundled libgd relies on zlib; we're setting up the library, but
don't check for the header (which is included by gd_gd2.c), which may
pass configure, although the build fails.  We better fail early if the
required header is not available.

Author: cmb69

ext/gd/config.w32

@@ -16,7 +16,8 @@ if (PHP_GD != "no") {
 		CHECK_HEADER_ADD_INCLUDE("png.h", "CFLAGS_GD", PHP_GD +  ";" + PHP_PHP_BUILD + "\\include\\libpng12")) &&
 		(CHECK_LIB("libiconv_a.lib;libiconv.lib", "gd", PHP_GD) || CHECK_LIB("iconv_a.lib;iconv.lib", "gd", PHP_GD)) &&
 		 CHECK_HEADER_ADD_INCLUDE("iconv.h", "CFLAGS_GD", PHP_GD) &&
-		SETUP_ZLIB_LIB("gd", PHP_GD)
+		SETUP_ZLIB_LIB("gd", PHP_GD) &&
+		CHECK_HEADER_ADD_INCLUDE("zlib.h", "CFLAGS", "..\\zlib;" + php_usual_include_suspects)
 		) {
 
 		if (CHECK_LIB("libXpm_a.lib", "gd", PHP_GD) &&