[Enhancement] Option to use gcc instead of AppleClang

[terabyte25]

In order to compile OpenMW with gcc, dependencies must be compiled with gcc as well due to linking errors and the compiler not being able to understand the argument "--stdlib"

[nikolaykasyanov]

Thanks for suggestion! My only concern is that there are some Objective-C++ files in OSG (and OpenMW itself), how well does GCC deal with Apple's Objective-C SDKs and Objective-C in general?

[terabyte25]

@nikolaykasyanov https://stackoverflow.com/questions/42308511/how-to-compile-objective-c-in-gcc
I am not sure about performance; but gcc can definitely compile it.

Most of them are for iOS use I think. Unless someone wants to write an objective c++ wrapper for OpenMW to port it to iOS platforms, it won't really be of any use.

Also OpenMW does not have objective c of any kind. OSG does though.

[nikolaykasyanov]

@terabyte25 thanks!

I've played with GCC a bit and it seems to be supporting macOS-specific stuff pretty well. I'd happily accept a PR making it possible to use GCC.