Don't accidently include both math.msvc.inl and math.gcc_builtin.inl.

[jeffdav]

Issue #, if available: #1155

Description of changes:
Add additional check to make sure we're not including both math.msvc.inl and math.gcc_builtin.inl.

Note: I'm not 100% sure this is the most correct solution, so I'd love feedback and I'm happy to iterate.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[waahm7]

Looking into AWS_HAVE_MSVC_INTRINSICS_X64, it will be only set if compiler is MSVC. We should not be setting MSVC and (GCC or Clang) at the same time.

[jeffdav]

I agree. I think the problem here is it gets built with MSVC, because I use your CMakeLists via ExternalProject. But we're not using the C modules directly; we're using AwsSdkSwift. And when I build that with CMake as part of the rest of the project the headers get imported as a module by clang, and that's where I start getting duplicate symbol errors.