Support GCC compiler as an option

[AaronRobinsonMSFT]

At present the compile on non-Windows platforms is hardcoded to clang. This should be configurable.

DNNE/src/msbuild/DNNE.BuildTasks/macOS.cs

Lines 53 to 54 in 66ec1e9

	command = "clang";
	commandArguments = compilerFlags.ToString();

[AaronRobinsonMSFT]

Support for GCC isn't explicitly added here. It was confirmed the generated command line arguments for the clang scenario also work with GCC. This scenario can be achieved by using the DnneCompilerCommand MSBuild property to provide the gcc command and override the default clang for non-Windows scenarios.