* 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

* improved compatibility with pure core profiles
* minor typos

* Added support for direct state access based uniform setting in the ShaderProgram
* functions from the EXT_direct_state_access extension can get emulated if the extension is not present
  in this case ACGL will map the simulated versions automatically to the *EXT function pointers
  so a programmer can just call the *EXT versions and doesn't have to care if the machine the code
  will get run on does support this extension (as long a simulated version already exists)
* not all functions have emulated version right now, so check if an alternative exists before relying on
  functions from this extension in ACGL lib or implement the missing simulation function to support
  machines without this extension
* all glProgramUniform functions can be emulated (plus some others that were quick to implement)
* dummy functions for all other simulation functions exist

* OpenGL error checking level can now be set via cmake
* OpenGL header configuration works better with external defines via cmake
* OpenGL version gets checked at runtime in the ACGL::init function and complains it the version it was compiled for is not present