fix basic variables not being initialized any more

QMAKE_INTERNAL_INCLUDED_FEATURES is added even before loading the first
prf file, so the check for an empty value map would never trigger. use a
flag variable instead.

Change-Id: If0af31d8466eadb90eca2a2e4024eff03b54e3b1
Reviewed-by: hjk <qthjk@ovi.com>