build-sys: pylibmount will not work when python3 is present

build-sys: pylibmount will not work when python3 is present

While compiling with up to date Archlinux I notice the recently added
pylibmount does not link correctly.  Failures inform fundamental types
such as PyFileObject and PyFile_Type being missing.

It seems automake AM_PATH_PYTHON will prefer the python in path, which
for this distribution right now is python3.  As some sort of go-around
one can install older python, and symlink it to earlier in PATH lookup.

$ ln -s /usr/bin/python2 $HOME/bin/python
$ export PATH=$HOME/bin:$PATH

Signed-off-by: Sami Kerola <kerolasa@iki.fi>