Remove pthread.h from janet.h

Should make janet a bit easier to use. Also changes the
header to not expose the size of native mutexes and rwlocks, except
with janet_os_mutex_size and janet_os_rwlock_size.

src/boot/boot.janet

@@ -45,6 +45,7 @@
 (defn defmacro :macro
   "Define a macro."
   [name & more]
+  (setdyn name @{}) # override old macro definitions in the case of a recursive macro
   (apply defn name :macro more))
 
 (defmacro as-macro