build: Link game, cgame, ui modules to $(LIBS)

[smcv]

All three modules call mathematical functions like atan2(). On glibc
systems, when compiled to native code with an ordinary C compiler
(as opposed to bytecode from q3lcc), they get the definition of those
functions from libm.

Until now, they were not explicitly linked to libm, and instead relied
on the fact that libm is linked into the main executable. However,
doing it this way defeats glibc's symbol-versioning mechanisms, and
can fail in some situations, in particular binaries built with
-ffast-math on a pre-2.31 version of glibc (where atan2() resolves to
__atan2_finite()), and used without recompilation on a post-2.31 version
of glibc (where __atan2_finite() has become a deprecated hidden symbol
that is only available as a versioned symbol).

When building shared libraries and loadable modules, it's most robust
to link them explicitly to their dependencies, as is done here.
$(LIBS) also includes -ldl, which is unnecessary but harmless.

---

Or I could separate out $(LIBM) (defined to -lm or empty, depending on platform) from $(LIBS), if preferred; please let me know.

[smcv]

I have versions of this patch for OpenArena and iortcw, which I'll send to their respective developers at some point, but I wanted to see whether there were comments on the best approach to this in ioquake3 first.

@ensiform, FYI: OpenJK doesn't seem to need a similar change, because all your linking is done with the C++ compiler, which pulls in libm anyway.

[smcv]

Or I could separate out $(LIBM) (defined to -lm or empty, depending on platform) from $(LIBS), if preferred; please let me know.

If people would prefer this, please say.