In jam, unset variables aren't equal to 0

Make our Werror check work again with gcc.

build/jam/ArchitectureRules

@@ -498,7 +498,7 @@ rule ArchitectureSetupWarnings architecture
 	rule EnableWerror dirTokens : scope {
 		# Clang gives way more warnings than GCC, so that code won't compile
 		# -Werror when using Clang.
-		if $(HAIKU_CC_IS_CLANG_$(architecture)) = 0 {
+		if $(HAIKU_CC_IS_CLANG_$(architecture)) != 1 {
 			SetConfigVar WARNINGS : HAIKU_TOP $(dirTokens) : treatAsErrors
 				: $(scope) ;
 		}