driconf: Add allow_glsl_builtin_variable_redeclaration option

driconf: Add allow_glsl_builtin_variable_redeclaration option

This option will allow GLSL builtins to be redeclared verbatim (e.g.
redeclaring "in int gl_VertexID" in a vertex shader). This is not strictly
valid and would normally fail to compile, but some applications (such as
newer Techland ports) do it and need more leniency.

v2 (Samuel Pitoiset):
    - Rename allow_glsl_builtin_redeclaration ->
      allow_glsl_builtin_variable_redeclaration

Signed-off-by: John Brooks <john@fastquake.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>