util: do not strip /dev prefix twice
build-sys: do not include id128-constants.h in the dist archive

File src/python-systemd/id128-constants.h is auto generated and its generation
does not require special tools, only sed.  There is no point in bundling it in
the distribution archive, so let's mark it as nodist_ to have it excluded.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=80006

Tested: Successfully ran "make dist" after ./configure --without-python.
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
build-sys: configure --with-python when running distcheck

Python support is pretty much essential to create man pages, so we should make
sure that distcheck will request it during configure.

Tested: Successfully ran "make distcheck" and confirmed --with-python was
present in the ./configure run inside the unpacked distribution directory.
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
build-sys: check that compat-libs are enabled for "make dist"

Running "make dist" requires --enable-compat-libs since DIST_SOURCES will list
generated files such as libsystemd-daemon.c.

Tested:
  $ ./configure && make && make dist
  *** compat-libs must be enabled in order to make dist
  make: *** [dist-check-compat-libs] Error 1
build-sys: fail if python support requested but not found