CMake doesn't check for libglu & libgl

[florczakraf]

When development package for libglu is not available CMake run passes, but we get an error during glew compilation:

In file included from /home/rf/proj/itgmania/extern/glew/src/glew.c:34:
/home/rf/proj/itgmania/extern/glew/include/GL/glew.h:1219:14: fatal error: GL/glu.h: No such file or directory
 1219 | #    include <GL/glu.h>
      |              ^~~~~~~~~~
compilation terminated.
make[2]: *** [extern/CMakeFiles/glew.dir/build.make:76: extern/CMakeFiles/glew.dir/glew/src/glew.c.o] Error 1

and when libgl libraries are missing, it fails at linking stage:

[ 70%] Linking CXX executable /home/rf/proj/itgmania/itgmania
/usr/bin/ld: CMakeFiles/ITGmania.dir/RageDisplay_OGL.cpp.o: in function `RageDisplay_Legacy::Init(VideoModeParams const&, bool)':
RageDisplay_OGL.cpp:(.text+0x1ab8): undefined reference to `gluGetString'
/usr/bin/ld: CMakeFiles/ITGmania.dir/RageDisplay_OGL.cpp.o: in function `SetupExtensions()':
RageDisplay_OGL.cpp:(.text+0x2f27): undefined reference to `gluGetString'
/usr/bin/ld: CMakeFiles/ITGmania.dir/RageDisplay_OGL.cpp.o: in function `RageDisplay_Legacy::CreateTexture(RagePixelFormat, RageSurface*, bool)':
RageDisplay_OGL.cpp:(.text+0xa232): undefined reference to `gluBuild2DMipmaps'
/usr/bin/ld: RageDisplay_OGL.cpp:(.text+0xa256): undefined reference to `gluErrorString'
/usr/bin/ld: RageDisplay_OGL.cpp:(.text+0xa277): undefined reference to `gluErrorString'
collect2: error: ld returned 1 exit status
make[2]: *** [src/CMakeFiles/ITGmania.dir/build.make:6696: /home/rf/proj/itgmania/itgmania] Error 1
make[1]: *** [CMakeFiles/Makefile2:3294: src/CMakeFiles/ITGmania.dir/all] Error 2
make: *** [Makefile:156: all] Error 2

see also #240