add support for PKGCONFIG_PRIVATE

add support for PKGCONFIG_PRIVATE

this does the same as PKGCONFIG, only that the libraries end up in
LIBS_PRIVATE, not LIBS, which means they don't end up in prl files.
in the vast majority of cases, the user should use this new variable.

Change-Id: Icb57a1166f1896f51284c64e4047cfc79410e73a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>