glsl2: Conditionally define preprocessor tokens for optional extensions

The only optional extension currently supported by the compiler is
GL_EXT_texture_array.

src/glsl/glcpp/glcpp.c

@@ -68,16 +68,13 @@ load_text_file(void *ctx, const char *file_name)
 	return text;
 }
 
-int
-preprocess(void *talloc_ctx, const char **shader, char **info_log);
-
 int
 main (void)
 {
 	void *ctx = talloc(NULL, void*);
 	const char *shader = load_text_file(ctx, NULL);
 	char *info_log = talloc_strdup(ctx, "");
-	int ret = preprocess(ctx, &shader, &info_log);
+	int ret = preprocess(ctx, &shader, &info_log, NULL);
 
 	printf("%s", shader);
 	fprintf(stderr, "%s", info_log);