I tried using glMinMax. Unfortunately I always get an error during compilation that the function was not found. All necessary libraries/dlls are bound and the function exists in the used GL-header-files (glext.h).
I do not think it is a problem with the GL-extensions due to the fact that it is a compilation error.
Can anybody help me?

Thank you!

Yes it is related to GL-extensions if you code for Windows. RTFAQ :

Read the part about extensions, and the links provided. Search for wglGetProcAddress.