Provide an option for using libobjc2 on Linux

[danielnachun]

We've recently worked on packing nu for Linux in Homebrew: Homebrew/homebrew-core#104928. We are using libobcj2 to provide a more modern Objective-C runtime than the one that comes with GCC. Nu builds with no issues using libobjc2, but it does require us to manually remove the -DGNU_RUNTIME=1 and -fgnu-runtime flags from the Nukefile because they are hardcoded when building on Linux.

Could a flag or environment variable be added that would allow the user to skip these flags, even if they are enabled by default? This is more convenient than having manually edit the file.