build-sys: check that python is enabled for "make dist"

build-sys: check that python is enabled for "make dist"

Running "make dist" requires Python support since some of the man page sources
(such as man/systemd.index.xml and man/systemd.directives.xml) are generated by
Python scripts, so break "make dist" and give an useful error message when
Python or the Python lxml module is not available.

Tested:
  $ ./configure --without-python && make && make dist
  *** python and python-lxml module must be installed and enabled in order to make dist
  make: *** [dist-check-python] Error 1