On install, merge janetconf.h into janet.h

This results in a cleaner amalgmated build

tools/patch-header.janet

@@ -0,0 +1,12 @@
+# Patch janet.h 
+
+(def [_ janeth janetconf output] (dyn :args))
+
+(def- replace-peg
+  (peg/compile
+    ~(% (* '(to `#include "janetconf.h"`)
+           (constant ,(slurp janetconf))
+           (thru `#include "janetconf.h"`)
+           '(any 1)))))
+
+(spit output (first (peg/match replace-peg (slurp janeth))))