The gresource code uses libelf if available but that also depends on mmap but isn't
guarded with HAVE_MMAP. This can make the build fail under MSYS2 where a mingw version
of libelf exists but there is no mmap.

Instead of guarting the libelf code with HAVE_LIBELF add a new macro named USE_LIBELF
which is only defined if libelf and mmap support are available.

Also install the mingw libelf version for CI so we catch similar errors in the future.