Revert "exit with error if lalxml and condor are both enabled"

This reverts commit 0ca6d02cff6af93561be6bc3be7cf1443a2b106c.
If using libxml2.0, ensure -llzma is linked against if available

- Fixes bug in libxml2.0 pkg-config file on Wheezy:
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=697382