There are many cl-specific features used in the source code: intrinsics,
pragmas, resource files, message files, extra data sections, stdcall
imports, etc., beyond just compiler and linker flags and differences in
compiler warnings. There used to be inline assembly but to support 64-bit
that's all been separated and will compile with either gas or masm. If
you'd like to add support for mingw, we'd be happy with such a
contribution, but it would not be a tiny change.
- Derek