Description of problem: Please update to mingw64-gtksourceview5 for cross-compilation. This might help with compilation flags, patches, etc.: https://packages.msys2.org/base/mingw-w64-gtksourceview5
As already discussed on fedora-mingw, the best way to make this happen is submitting a package for review. Also, this is not a bug related to mingw-gtksourceview3.