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.
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