Work on windows installer.

We will probably shift to NSIS as the default
installation method for windows. Shipping around a
single binary just doesn't cut it if we want to be able
to reliably use tools like `jpm` to build things.

src/boot/boot.janet

@@ -1605,13 +1605,14 @@
 (var module/*syspath*
   "The path where globally installed libraries are located.
   The default is set at build time and is /usr/local/lib/janet on linux/posix, and
-  on Windows is C:/Janet/Library."
+  on Windows is the empty string."
   (or (process/opts "JANET_PATH") ""))
 
 (var module/*headerpath*
   "The path where the janet headers are installed. Useful for building
-  native modules or compiling code at runtime."
-  (process/opts "JANET_HEADERPATH"))
+  native modules or compiling code at runtime. Default on linux/posix is
+  /usr/local/include/janet, and on Windows is the empty string."
+  (or (process/opts "JANET_HEADERPATH") ""))
 
 # Version of fexists that works even with a reduced OS
 (if-let [has-stat (_env 'os/stat)]

src/mainclient/init.janet

@@ -12,6 +12,7 @@
   (var *compile-only* false)
 
   (if-let [jp (os/getenv "JANET_PATH")] (set module/*syspath* jp))
+  (if-let [jp (os/getenv "JANET_HEADERPATH")] (set module/*headerpath* jp))
 
   # Flag handlers
   (def handlers :private