assimp: Clean and document buildsystem, prepare for unbundling

- Improve the SCsub to allow unbundling and remove unnecessary code.
- Move files around to match upstream source.
- Re-sync with upstream commit 308db73d0b3c2d1870cd3e465eaa283692a4cf23
  to ensure we don't have local modifications.
- Doesn't actually build against current version 5.0.1 due to the lack
  of the new ArmaturePopulate API that Gordon authored. We'll have to
  wait for a public release with that API (5.1?) to enable unbundling.

(cherry picked from commit 9d8a9ea826)

platform/server/detect.py

@@ -159,6 +159,10 @@ def configure(env):
             sys.exit(255)
         env.ParseConfig('pkg-config bullet --cflags --libs')
 
+    if False:  # not env['builtin_assimp']:
+        # FIXME: Add min version check
+        env.ParseConfig('pkg-config assimp --cflags --libs')
+
     if not env['builtin_enet']:
         env.ParseConfig('pkg-config libenet --cflags --libs')