[workerd-cxx] __WORKERD_CXX__ define to identify when workerd-cxx fork is in use (#8)

Author: mikea

include/cxx.h

@@ -23,6 +23,10 @@
 #include <sys/types.h>
 #endif
 
+#ifndef __WORKERD_CXX__
+#define __WORKERD_CXX__
+#endif
+
 namespace rust {
 inline namespace cxxbridge1 {