Use pkg-config to detect libxml2, falling back to xml2-config if the
former is not available.

This patch ensures Asterisk continues to build on systems without
xml2-config installed.

The patch also updates the associated 'configure' files.

ASTERISK-29970 #close

Change-Id: I3c90dfe0b0590486cbb8e6d426a7c5c4199410c0