Refactor our build skipping logic

Move the logic to mkspecs to keep the .pro files robust.

Add the basic infrastructure for config.tests, the first one
of which will consistently ensure we have libcap on linux.

Change-Id: Iee4207e747e589ba67d5353cb4c18f156e555c11
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>