xorg-x11-devel contains the symlinks 
/usr/X11R6/%{_lib}/libGLU.so
%{_libdir}/libGLU.so
but xorg-x11-devel doesn't contain any (direct) dependancy to pull in the target
of that symlink.  The same can be said for libGL, but that get's pulled
indirectly via xorg-x11-libs.

Not sure how best to handle this, but possibilities include:
1.  using file deps: -devel should
Requires: %{_x11libdir}/libGLU.so
Requires: %{_x11libdir}/libGL.so

2.  using explicit pkg deps: -devel should:
Requires: %{name}-Mesa-libGL = %{version]-%{release}
Requires: %{name}-Mesa-libGLU = %{version]-%{release}

3.  using virtual libGL/libGLU dep: -devel should
Requires: libGL.%{_target_cpu} libGLU.%{_target_cpu}
and -Mesa-libGL should
Provides: libGL.%{_target_cpu} = %{libGL_version}
and -Mesa-libGLU should
Provides: libGLU.%{_target_cpu} = %{libGLU_version}

I'd say 1 appears to be simplest/sanest approach.