Use -std=gnu++11 on cygwin and mingw.

Without this common features like off_t and strdup are missing.
This should bring back those bots.

Configure bits by Meador Inge.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203701 91177308-0d34-0410-b5e6-96231b3b80d8