Updates required for building the SciTech SNAP version of Mesa. This includes:

Updates required for building the SciTech SNAP version of Mesa. This includes:

1. Changes to the main header files to exclude some stuff is __SCITECH_SNAP__
   is defined. The main GL/gl.h header file in SciTech SNAP is actually a wrapper
   that defines a bunch of necessary stuff and then included GL/gl_mesa.h which
   is the normal Mesa GL/gl.h header file renamed.

2. Changed APIENTRY * to APIENTRYP macros in the GL/gl.h and GL/glext.h header
   files. This will better support other compilers like IBM VisualAge C++. I
   added a basic macro for APIENTRYP to GL/gl.h (inside the !SNAP block), so
   existing code will compile the same, but when the SNAP version is being
   built we will use the correc definitions for the target compiler.

3. Changed a few more void * declarations to GLvoid *'s to avoid Open Watcom
   compiler complaints.

4. Updates the OPENGL_VERSION macro to include a patch number (set to 0 for
   now). Probably not necessary, but the original macro was wrong and we use
   that macro in our code.

5. Changed _tnl_end() to _tnl_end_ctx() to avoid conflicts with _tnl_End when
   using a compiler that has a case insensitive link.