glcpp: Don't use alternation in the lookahead for empty pragmas.

glcpp: Don't use alternation in the lookahead for empty pragmas.

We've found that there's a buffer overrun bug in flex that's triggered by
using alternation in a lookahead pattern.

Fortunately, we don't need to match the exact {NEWLINE} expression to
detect an empty pragma. It suffices to verify that there are no non-space
characters before any newline character. So we can use a simple [\r\n] to
get the desired behavior while avoiding the flex bug.

Fixes the regression of piglit's 17000-consecutive-chars-identifier test,
(which has been crashing since commit
04e40fd337a244ee77ef9553985e9398ff0344af ).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82472
Signed-off-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
CC: <mesa-stable@lists.freedesktop.org>