systemd-python: use modern C
build-sys: use pkg-config for python compilation flags

Python 2.7, and 3.2 and higher support querying compilation
flags through pkg-config. This makes python support follow
rules similar to various other optional compilation-time
libraries. New flags are called PYTHON_DEVEL_CFLAGS and
PYTHON_DEVEL_LIBS, because PYTHON (without _DEVEL), is
already used for the python binary name, and things would
be confusing if the same prefix was used for two things.
configure has --disable-python-devel to disable python modules.

One advantage is that CFLAGS for modules gets smaller:
- -I/usr/include/python3.3m -I/usr/include/python3.3m -Wno-unused-result -DDYNAMIC_ANNOTATIONS_ENABLED=1 -DNDEBUG -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -D_GNU_SOURCE -fPIC -fwrapv
+ -I/usr/include/python3.3m
as does LIBS:
- -lpthread -ldl -lutil -lm -lpython3.3m
+ -lpython3.3m

Support for Python 2.6 is removed, but can be easily
restored by using
PYTHON_DEVEL_CFLAGS="$(python2.6-config --cflags)",
etc., as ./configure parameters.

https://bugs.freedesktop.org/show_bug.cgi?id=57800
systemd-python: fix gcc warning

src/python-systemd/_reader.c: In function Reader_get_catalog:
src/python-systemd/_reader.c:912:53: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             assert(mid_len > l);
                            ^
logind: update state file after generating the session fifo, not before

https://bugs.freedesktop.org/show_bug.cgi?id=67273