[C++] Define MAKE=make -j cpus/2

So that sub-make works, although ninja won't be used.

Also define KATI bootstrap variable to make it easier to
determine if we are running make or kati.
diff --git a/bootstrap.go b/bootstrap.go
index cdacd8c..9ef396a 100644
--- a/bootstrap.go
+++ b/bootstrap.go
@@ -30,6 +30,7 @@
 MAKE?=kati
 # Pretend to be GNU make 3.81, for compatibility.
 MAKE_VERSION?=3.81
+KATI?=kati
 SHELL=/bin/sh
 # TODO: Add more builtin vars.