nxagent: implement RPATH/RUNPATH override for libX11.

As discussed in ArcticaProject/nx-libs#610, we need a way to load
libNX_X11 as a fake version of libX11 into nxagent's address space.

For doing so, we have to link against libX11 at build-time and specify
a(n) RPATH/RUNPATH to our libX11 -> libNX_X11 compat symlinks.

This commit is essentially doing a cross between options 3 and 4
discussed in the provided GH issue.

We link libX11 early (before specifying -Lcompat_symlinks_dir), so that
the linker finds the "real" libX11 version with the libX11 SONAME. This
leads to our binary depending upon both libX11 and libNX_X11 SONAMEs. We
already always added RPATH/RUNPATH, but these values are not passed down
to dependent libraries.

What happens at run-time is that the loader searches for a libX11
SONAME, takes RPATH/RUNPATH into account and loads our libNX_X11 library
instead via the compat symlinks. This satisfies the libX11 SONAME and
dependent libraries will *not* load the system libX11 version again.

Debian's dpkg-shlibdeps isn't quite happy about this mismatching SONAME
situation, so instruct to look the other way while we're minding our
business.

Fixes: ArcticaProject/nx-libs#610

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