Add checks to configure for sufficiently modern host compilers. This
requires Clang 3.1 or GCC 4.7. If the compiler isn't Clang or GCC, we
don't try to do any sanity checking, but this give us at least
a reasonable baseline of modern compilers.

Also, I'm not claiming that this is the best way to do compiler version
tests. I'm happy for anyone to suggest better ways of doing this test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199182 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the last weird subproject, 'privbracket'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199183 91177308-0d34-0410-b5e6-96231b3b80d8
[PM] Fix stale header blocker, found by Duncan Smith in code review!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199185 91177308-0d34-0410-b5e6-96231b3b80d8