I was trying to cross compile SDL. Many dependencies work, but I ended seeing
libX11 not ready for cross compilation. Other xorg libraries cross-compile
well. libX11 may need a small patch. The problem is the usual "configure test
cannot be run in cross compilation", so the configure script halts.
I made the pkgconfig expression always return buildDrv, as I think it rarely
will be needed as buildInput. So to avoid rewriting all its mentions to use
it as buildNativeInput, I prefered this small change.
svn path=/nixpkgs/branches/stdenv-updates/; revision=18500
ca5d91aa60