chromium: Selectively link bundled libraries.

So far we just copied everything from source.* into the build directory
during the prePatch. This somewhat defeated the purpose of the source
splitup because it involved more I/O than just unpacking the entire
source tree.

Now, we're selectively *symlinking* the bundled sources into the build
directory. Even that isn't perfect because in the end we'd just
reference foreign derivations and we're done. But for now, this gets us
at least prepared for a massive reduction of compile time.

Unfortunately, gyp's behaviour when it comes to symlinks is quite
painful to come by, so we need to fix a few references to use absolute
paths.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>