daemon: Use pkg-config to locate Augeas CFLAGS / libraries.

daemon: Use pkg-config to locate Augeas CFLAGS / libraries.

Augeas 0.10 depends on libxml2, so this is now required in the
appliance (in fact, it was already present).

However this exposed two bugs:

(1) In libguestfs we use a home-brewed recipe for Augeas flags,
resulting in this error:
/usr/include/augeas.h:24:25: fatal error: libxml/tree.h: No such file or directory

(2) Augeas's own augeas.pc didn't include the libxml2 flags, so
it was broken.  This requires a patch to Augeas 0.10, see:
https://www.redhat.com/archives/augeas-devel/2011-December/msg00008.html

Change to using pkg-config to detect Augeas.  It is still an optional
library.