Bug 22248 – ImportC: Specify C compiler when multiple options are available

For instance, on Linux, dmd can link with code built with both gcc and clang (and likely others). If D will compile C code, and there may be differences between the various compilers (e.g. bitfield handling), the ImportC compiler should select a default compiler for that platform. Obviously, ldc should match llvm, and gdc should match gcc. But DMD doesn't always have an equivalent C compiler. Optionally, it may be useful (if all the code is going to be inside the front end anyway) to specify an expected compatible compiler.

This is already taken care of by the target options.