rrdtool: Build against glib2 and libxml2 from the BCR

Both libraries are now built from source and linked statically instead of
being resolved against whatever the distro ships, which makes the build
independent of the host's glib and libxml2 versions.  glib is
LGPL-2.1-or-later, which is unproblematic here because direct calls to rrd
were removed from the non-free software some time ago.

Change-Id: I880418b6a82eee98b65ea913460488096a6a6964