Append to CMAKE_PREFIX_PATH instead setting it outright.

Also, don't set it in the FindXxx cmake scripts--it's unnecessary, and
it resets the value.
Drop the deb-src for clang.

The llvm.org/apt repo currently has an issue with the source packages,
and we don't need it on Travis anyways.
Extract path.c from misc1.c
Move remove_duplicates to garray.c
foo