clang:

- /usr/local/bin/clang (front-end) doesn't search /usr/local/libexec for
the clang-cc backend.

- library search order doesn't match base compilers (/usr/local/include
before /usr/include).

- target-specific definitions (__OpenBSD__ etc); see
tools/clang/lib/Basic/Targets.cpp

- scan-build'ing an autoconf project fails

