Proper detection and handling of RHEL and variants.

- Don't consider "/etc/lsb-release" to be Ubuntu only.
- Detect SL, too.
- Only add "--no-add-needed" for RHEL7 (or Fedora), not for RHEL6
  (that's what the compilers shipped with RHEL do).
- removed RHEL4 which is now four years past EOL and certainly incapable
  of building or running any recent version of llvm/clang.

llvm-svn: 268914