gresource-tool: fix build in case libelf is available on Windows. Fixes #1466
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.