make qmakespecs announce the compiler family

make qmakespecs announce the compiler family

on the way to eliminate scoping based on the spec.

gcc and msvc go as such into CONFIG, the other ones get the vendor
prefixed, as most are mostly unknown and thus likely to clash with
users' flags.

Change-Id: Ie622f53d90e96dbf05ce7d8c638cd355f04fa20c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>