cmake: Don't do the libstdc++ version check when clang simulates MSVC

If we're using clang-cl, that's a pretty good indication that we're
going to use MSVC's STL.

This simplifies the clang-cl ninja self-host configuration down to:
CC=clang-cl CXX=clang-cl cmake .. -GNinja

Modified version of zturner's patch:
Differential Revision: http://reviews.llvm.org/D7824

llvm-svn: 230239