CGAL: change source from tar.xz to tar.bz2

Darwin platforms does not have `xz' available by default, fallback
to tar.bz2.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
llvm: enable O4 only on Linux platform

LLVM/Clang needs `binutils' headers to genreate LLVMgold plugin
for ld.gold. On Linux headers are installed under ${GCC_ROOT}/include
while on Darwin they are not. They seems to be available under

    ${GCC_ROOT}/lib/gcc/<triplet>/<gcc_version>/plugin/include

on both platforms.

Futher investigation is needed.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>