Replace COSMO define with _COSMO_SOURCE

This change might cause ABI breakages for /opt/cosmos. It's needed to
help us better conform to header declaration practices.

third_party/quickjs/qjsc.c

@@ -636,7 +636,7 @@ int main(int argc, char **argv)
                 "\n"
                 );
     } else {
-#ifndef COSMO
+#ifndef _COSMO_SOURCE
         fprintf(fo, "#include <inttypes.h>\n"
                 "\n"
                 );