That is a much better place for it. The gb_C(XX)FLAGS don't get passed
on to "external" modules, so they should not contain flags that are
essential for the compilation to work at all. I think.

At least for me, in my MacOSX SDK 10.4 -based build tree, using Xcode
3 installed in /Xcode3, on OS X 10.8, the -isysroot is essential for
liborcus to compile.

Change-Id: I1f11c690585e4ba512eb87d2fddf11872b91dd57