glcpp: Raise error when modulus is zero

For example, this now raises an error:
   #define XXX 1 / 0

Fixes bug: https://bugs.freedesktop.org//show_bug.cgi?id=33507
Fixes Piglit test: spec/glsl-1.10/preprocessor/modulus-by-zero.vert

NOTE: This is a candidate for the 7.9 and 7.10 branches.

src/glsl/glcpp/glcpp-parse.y

@@ -388,7 +388,12 @@ expression:
 		$$ = $1 + $3;
 	}
 |	expression '%' expression {
-		$$ = $1 % $3;
+		if ($3 == 0) {
+			yyerror (& @1, parser,
+				 "zero modulus in preprocessor directive");
+		} else {
+			$$ = $1 % $3;
+		}
 	}
 |	expression '/' expression {
 		if ($3 == 0) {