Nasty build hack:
When setting SHA1 to ppc in CMakeLists.txt, after running initial CMake,
copy src/ppc/sha1ppc.S.obj to build/CMakeFiles/git2.dir/src/ppc/
Add CMakeFiles/git2.dir/src/ppc/sha1ppc.S.obj to the list in build/CMakeFiles/git2.dir/link.txt