build-sys: add verbosity to python-lxml detection in ./configure

build-sys: add verbosity to python-lxml detection in ./configure

Be verbose when checking if Python module lxml is available.  Also warn that
Python support will be disabled when the lxml module is not present.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80005
Tested:
- Without python-lxml package installed:
  $ ./configure
  checking for python extension module directory... ${exec_prefix}/lib64/python2.7/site-packages
  checking for python lxml module... no
  configure: WARNING: *** python support requires python-xml module installed

- With python-lxml package installed:
  $ ./configure
  checking for python extension module directory... ${exec_prefix}/lib64/python2.7/site-packages
  checking for python lxml module... yes
  checking for PYTHON_DEVEL... yes
  ...
          Python:                  yes
          Python Headers:          yes