configure: improve pkg-config support

This adds helper functions for checking packages with pkg-config
and managing the associated flags.

Note that pkg-config use is still discouraged due to widespread
poor practices resulting in broken flags in many situations.  A
few badly designed packages require flags only obtainable using
pkg-config, and these functions are intended for those cases.

Signed-off-by: Mans Rullgard <mans@mansr.com>