libgl Merge Request #1: libGL: glClearDepth() defined as macro gives problems when kos/addons has another gl.h include (merged)

As stand-alone solution, there is no problem with kos & kos-ports:

• we can use glClearDepth (1.0f) in a program,
• the kos-ports libGL include file "gl.h" will transform that to glClearDepthf (1.0f),
• and it will use glClearDepthf (1.0f) in the libGL library.

Once we start adding extra kos/addons (eg: GLdc), which have glClearDepth() defined as a real function in their "gl.h" include file, the problem arises:

• we still can use glClearDepth (1.0f) in a program,
• but because of include folder priorities, the include file "gl.h" in kos/addons will be used,
• it will hence not transform glClearDepth() into glClearDepthf,
• and if you then want to compile a program based on libGL (eg: all kgl examples), it will give an error & block the build, because glClearDepth() does not appear as a function in libGL.

There are several ways to solve this, I tried to keep it in lign with the rest of libGL
(and not eg define the function with Glclampd / GLdouble as the OpenGL spec specifies), since those are not used elsewhere in libGL, and would be mapped to floats anyways in libGL).