glsl/glcpp: Allow single-line comments immediately after #define

glsl/glcpp: Allow single-line comments immediately after #define

We were already correctly supporting single-line comments in case like:

	#define FOO bar // comment here...

The new support added here is simply for the none-too-useful:

	#define // comment instead of macro name

With this commit, this line will now give the expected "#define without
macro name" error message instead of the lexer just going off into the
weeds.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>