conf: use better clang detection

Handle case where compiler isn't called 'clang'. This is slightly
modified from the version here:
http://src.chromium.org/svn/branches/1312/src/third_party/mesa/MesaLib/configure.ac

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
Merge pull request #829 from ceph/port/detect-clang

conf: use better clang detection