add cache() function

add cache() function

this function adds the current contents of the named variable to the
cache. this comprises populating base_vars and appending an assignment
to .qmake.cache. if no cache file exists yet, it will be created in the
current output directory (and inherited by subdirs projects). if called
without a variable name, only create the cache file if missing.

Change-Id: I1e81c2238aa6a5817a6ebbfb022e2b995c349363
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>