portability fixes

diff --git a/libasn1parser/asn1p_expr2uclass.h b/libasn1parser/asn1p_expr2uclass.h
index 8bb4f63..1fb6dbb 100644
--- a/libasn1parser/asn1p_expr2uclass.h
+++ b/libasn1parser/asn1p_expr2uclass.h
@@ -1,6 +1,10 @@
 #ifndef	ASN1_PARSER_EXPR2UCLASS_H
 #define	ASN1_PARSER_EXPR2UCLASS_H
 
+#ifndef	__GNUC__
+#define	__attribute__(x)	/* unused */
+#endif
+
 static int expr_type2uclass_value[ASN_EXPR_TYPE_MAX]
 		__attribute__ ((unused)) = {
 	[ ASN_BASIC_BOOLEAN ]		= 1,