glcpp: Only disallow #undef of pre-defined macros on GLSL ES >= 3.00 shaders

glcpp: Only disallow #undef of pre-defined macros on GLSL ES >= 3.00 shaders

Section 3.4 (Preprocessor) of the GLSL ES 3.00 spec says:

   It is an error to undefine or to redefine a built-in (pre-defined)
   macro name.

The GLSL ES 1.00 spec does not contain this text.

Section 3.3 (Preprocessor) of the GLSL 1.30 spec says:

   #define and #undef functionality are defined as is standard for C++
   preprocessors for macro definitions both with and without macro
   parameters.

At least as far as I can tell GCC allow '#undef __FILE__'.  Furthermore,
there are desktop OpenGL conformance tests that expect '#undef
__VERSION__' and '#undef GL_core_profile' to work.

Fixes:

    GL45-CTS.shaders.preprocessor.definitions.undefine_version_vertex
    GL45-CTS.shaders.preprocessor.definitions.undefine_version_fragment
    GL45-CTS.shaders.preprocessor.definitions.undefine_core_profile_vertex
    GL45-CTS.shaders.preprocessor.definitions.undefine_core_profile_fragment

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: mesa-stable@lists.freedesktop.org