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.

src/glsl/glcpp/glcpp-parse.y

@@ -241,6 +241,16 @@ control_line:
 			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);

src/glsl/glcpp/tests/080-if-without-expression.c.expected

@@ -1,3 +1,6 @@
 0:2(1): preprocessor error: #if with no expression
-
  
+
+
+
+