Compatibility improvements

* Some GL 2.1 compatibility improvements:
  * On GL 2.1 the shaderfiles are read from a different folder by default
  * 3.0+ functions are not compiled for 2.1 any more
  * The RenderObject is not 2.1 compatible, so it's not compiled for old systems
    atm. A port could be possible.
  * ACGL compiles for GL 2.1 now
* Added compile warning in case the desired OpenGL version is not defined by
  the build system (which could screw up conditional compile)
* Minor fixes