Various fixes for both meson and autoconf for building Xwayland without GLamor nor libdrm

While reviewing !248 (closed), Emil (@evelikov) pointed out that Xwayland now requires libdrm.

That was a bit of a surprise to me, because if building without GLamor support (which is supported by Xwayland) and without libdrm (which is possible according to the build system), there is no reason to link against libdrm as it is not required nor used.

Looking at the build system, I found the problem which is present in both autoconf and meson build systems, so I took a bit of time to try to fix that.

I think this is important, as long as we give the option to disable support for things like glamor, to actually take care of not over linking with libs that aren't required in that case.

The following series of patches aim at requiring and linking Xwayland with libdrm and epoxy only when this is needed.