runtime/c/setup.sh: use make -j to speed up compilation

The configuration script takes significantly longer to run than the compilation
of all the C code, which is a hint that is in need of a major simplification.

src/runtime/c/setup.sh

@@ -11,7 +11,7 @@ case "$command" in
     ./configure "$@"
     ;;
   build)
-    make "$@"
+    make -j "$@"
     ;;
   copy|install)
     make install "$@"