Add a comment in buildconf.py.am.in

Associated to multisite loading & context stuff. I choose to commit this
change in master because it is important to not break the fact that
the buildconf HAS TO be importable safely outside any context.

lodel/buildconf.py.am.in

 ##@brief Stores constant evaluated at build time
 #@note Populated by ./configure
+#@warning This file HAS TO be safely importable outside any lodel context
 
 PYMONGO=@PYMONGO@
 WEBUI=@WEBUI@