glcpp: Raise error if defining any macro containing two consecutive underscores

glcpp: Raise error if defining any macro containing two consecutive underscores

The specification reserves any macro name containing two consecutive
underscores, (anywhere within the name). Previously, we only raised
this error for macro names that started with two underscores.

Fix the implementation to check for two underscores anywhere, and also
update the corresponding 086-reserved-macro-names test.

This also fixes the following two piglit tests:

	spec/glsl-1.30/preprocessor/reserved/double-underscore-02.frag
	spec/glsl-1.30/preprocessor/reserved/double-underscore-03.frag

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Carl Worth <cworth@cworth.org>