On nix platforms, patch jpm with path information.

This means we no longer need to guess paths after install.
Custom directory layouts can now be better supported at install
time without need for environment variables.

jpm

@@ -26,21 +26,29 @@
     (def i (last (string/find-all sep exe)))
     (slice exe 0 i)))
 
-(def JANET_MODPATH (or (os/getenv "JANET_MODPATH") (dyn :syspath)))
+###START###
+# Overriden on some installs.
+(def- install-paths 
+  {:headerpath (os/realpath (string exe-dir "/../include/janet"))
+   :libpath (os/realpath (string exe-dir "/../lib"))
+   :binpath exe-dir})
+###END###
 
 # Default based on janet binary location
 (def JANET_HEADERPATH (or (os/getenv "JANET_HEADERPATH")
-                          (string exe-dir "/../include/janet")))
+                          (get install-paths :headerpath)))
 (def JANET_LIBPATH (or (os/getenv "JANET_LIBPATH")
-                       (string exe-dir "/../lib")))
-
+                       (get install-paths :libpath)))
 # We want setting JANET_PATH to contain installed binaries. However, it is convenient
 # to have globally installed binaries got to the same place as jpm itself, which is on
 # the $PATH.
 (def JANET_BINPATH (or (os/getenv "JANET_BINPATH")
                        (if-let [mp (os/getenv "JANET_MODPATH")] (string mp "/bin"))
                        (if-let [mp (os/getenv "JANET_PATH")] (string mp "/bin"))
-                       exe-dir))
+                       (get install-paths :binpath)))
+
+# modpath should only be derived from the syspath being used or an environment variable.
+(def JANET_MODPATH (or (os/getenv "JANET_MODPATH") (dyn :syspath)))
 
 #
 # Utilities