build: further improve flags handling for cl

This allows us to build with cl (or rather clang-cl) by using the
correct spelling for `-include` (`/FI` for cl).  clang-cl and cl default
to C++11/C++14 as they support it rather than permitting an explicit
language standard.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290802 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed