Fix up PWM_fan

Use a custom setup script to check for the dependencies and pass along
the required compile flags to the module; also split the object
definitions for the target modules from their source so as to allow
#including them.

usermods/PWM_fan/library.json

@@ -0,0 +1,6 @@
+{
+  "name:": "PWM_fan",
+  "build": {
+    "extraScript": "setup_deps.py"
+  }
+}