Fixes #110.

python insists on adding . to python.path, so we always import the systemd
in the top-level directory instead of the one in build/ that we want. Let's
cd into docs/ first, so that we get the right module imported.

Also, replace sphinx-build calls with $(PYTHON) -m sphinx. This has the
advantage that once $(PYTHON) is set, the appropriate sphinx executable is
chosen.

I see no good reason to support old standards, when systemd itself
requires c99. Let's test with c99 in the CI.

Closes #62.

In systemd-233 the format of the constants file changed to use
SD_ID128_MAKE_STR macro and long lines are broken with '\'.
Doing this in sed is too anyoing — add a simple python script to
do the processing.

gcc warns about an include directive that shadows the system include
directory at high verbosity levels. Let's filter it out (pkgconfig
implements that) to reduce noise and also to make the command line
shorter.

This way we can compile against unreleased systemd:
make PYTHON=python3 INCLUDE_DIR=/home/zbyszek/src/systemd/src
(cd build/lib.linux-x86_64-3.5; systemd-socket-activate -E LD_LIBRARY_PATH=/home/zbyszek/src/systemd-master/.libs -l2000 -l127.0.0.1:2001 python3)