Port Mesa to build on a P64 platform (e.g., Win64). P64 platforms

Port Mesa to build on a P64 platform (e.g., Win64).  P64 platforms
use 64-bit pointers and 32-bit longs.  So, operations like casting pointers
to unsigned long and back to pointer won't work.  glheader.h now
includes files to define uintptr_t, which should instead be used for
this sort of operation.  It is an integer type that is the same size
as a pointer.