2.1.13 Begin to use PKG_CHECK_MODULES in configure.ac

This patch uses PKG_CHECK_MODULES to statically link some of the libraries used by shairport-sync. It is intended to make it easier to build in the buildroot system. While sufficient for that purpose, note that PKG_CHECK_MODULES is not used for checking all the libraries yet.

Functionality of shairport-sync is unchanged.