This patch contains the code generation scripts that convert the files
gl.spec, gl.tm, and enumext.spec into C code, as well as the CMake
directives to run the scripts.

Code generation occurs in two phases: first the script
glapi/parse_glspec.py converts the .spec and .tm files into an
intermediate JSON format, which is stored in glapi/glapi.json.  Then,
the script tests/util/gen_dispatch.py converts the JSON file into two
files, tests/util/generated_dispatch.c and
tests/util/generated_dispatch.h.

There are two motivations for breaking code generation into two phases:
(1) there are other code generation tasks we may add in the future
(e.g. generating the tests/util/piglit-util-enum.c file), and it would
be nice not to have to rewrite any parsing code when we do so.  (2) if
the GL consortium ever decides to change the format of gl.spec, gl.tm,
and enumext.spec, then we only have to change parse_glspec.py.

These code generation scripts can be run by invoking "make
piglit_dispatch_gen" from the toplevel source directory.

v2: Add clarifying comments, remove a bogus "print" statement, and
raise an exception if an unrecognized multiplicity is encountered.
Also, handle "reference" multiplicity.

v3: Add APIENTRY to stub functions.  Rename "near" and "far" to avoid
problems on MSVC.  Get rid of "__" in function names.  Rename
initialize_dispatch_pointers => reset_dispatch_pointers.  Add
gl_10x_version parameter to get_core_proc().  Output generated code
relative to CMAKE_BINARY_DIR so that it won't pollute source
directories when building out of tree.  Add .gitignore so that
generated files are ignored when building in tree.  Add GL_VERSION_*
defines.