build: Don't dist generated sources

Don't distribute the sources generated by gdbus-codegen or glib-mkenums.
These break the build on systems that are disimilar from the one that
'make dist' was run on.

Closes #399
configure.ac: Locate com.redhat.lvm2.xml interface during configure

Fixes #197

This is needed to prevent the previous patch from breaking 'make
distcheck' as $(datadir) was defined to a non-system value.