using pkg-config search as primary method with fallback to path lookup,
this may result in overlinking however if libraries require additional
include paths (as is the case with PulseAudio) with this change that will
not be a problem.
it will also reduce the lookups done during configuration/build since path
lookups will not be done, unless pkg-config lookup does not set the
required variables.
Signed-off-by: Ivailo Monev <xakepa10@laimg.moc>