glcpp: Add an explicit diagnostic for #if with no expression.

This is more clear than the previously-generated diagnostic which was
something confusing like "enexpected newline".

This change makse test 080-if-witout-expression.c now pass.
diff --git a/src/glsl/glcpp/glcpp-parse.y b/src/glsl/glcpp/glcpp-parse.y
index 795030e..0e0d9d4 100644
--- a/src/glsl/glcpp/glcpp-parse.y
+++ b/src/glsl/glcpp/glcpp-parse.y
@@ -241,6 +241,16 @@
 			parser->skip_stack->type = SKIP_TO_ENDIF;
 		}
 	}
+|	HASH_IF NEWLINE {
+		/* #if without an expression is only an error if we
+		 *  are not skipping */
+		if (parser->skip_stack == NULL ||
+		    parser->skip_stack->type == SKIP_NO_SKIP)
+		{
+			glcpp_error(& @1, parser, "#if with no expression");
+		}	
+		_glcpp_parser_skip_stack_push_if (parser, & @1, 0);
+	}
 |	HASH_IFDEF IDENTIFIER junk NEWLINE {
 		macro_t *macro = hash_table_find (parser->defines, $2);
 		talloc_free ($2);