Misc fixes and improvements for Requires and Requires.private

[Tollef Fog Heen]

]] Loïc Minier 

|  poke!  :-)

Sorry, been busy, but now it's xmas vacation, so I'm hoping to have a
little time for pkg-config again.

-- 
Tollef Fog Heen
UNIX is user friendly, it's just picky about who its friends are