scons: don't ignore PKG_CONFIG_LIBDIR environment variable

This is an attempt to fixup our scons pkg-config code, following
issue raised here:

https://github.com/Homebrew/homebrew-games/pull/165#issuecomment-61391362

scons/pkgconfig.py

@@ -6,6 +6,7 @@ def run_pkg_config(context, name):
     env = context.env
     try:
         env["ENV"]["PKG_CONFIG_PATH"] = os.environ.get("PKG_CONFIG_PATH")
+        env["ENV"]["PKG_CONFIG_LIBDIR"] = os.environ.get("PKG_CONFIG_LIBDIR")
         env.ParseConfig("pkg-config --libs --cflags --silence-errors $PKGCONFIG_FLAGS \"" + name + "\"")
         context.Log("Found '" + name + "' with pkg-config.\n")
         return True