Some linux distibutions (for example, Mageia 2, Fedora 17) ship Clang that is
essentially broken for the end user.  Clang can not find or compile libstdc++
headers.

The issue is that our configure prefers clang over gcc, thus selecting a broken
Clang when a working GCC is available.

Now we detect this issue by compiling a simple program.  If it does not
compile, configure stops with an error suggesting the user to select a
different compiler.

llvm-svn: 171975