build: Select the libboost_python matching the python version.

[BsAtHome]

Recent changes in Debian sid cause the wrong libboost_python to be selected in configure (313 instead of 314) causing a crash in milltask.
The python version is now extracted and used to test the "proper" library first before a generic test of which potential libraries are available on the filesystem.

[grandixximo]

Saw you flagged this as "a bit hackery". I think reusing what configure already has would help make it less so.

AX_PYTHON_DEVEL already extracts the version into $ac_python_version (e.g. 3.14) and builds PYTHON_LIBS=-lpython$ac_python_version from it. So instead of shelling out to $PYTHON_BIN again:

ax_python_lib="boost_python`echo $ac_python_version | tr -d .` boost_python"

No extra subprocess, and the boost suffix is guaranteed to match the libpython the build already links. The ordering idea itself is right (CMake derives the same python3xx name the same way). Does that feel less hackery to you?

[BsAtHome]

Well, yes... I was looking for it but overlooked it. Now it is used with the python method as fallback. Now it feels a lot more resilient.

BTW, your version also creates a sub-process. How else is tr run? ;-)

[grandixximo]

Yeah tr to get rid of the period is less extra lol
Can you backport/cherry-pick to 2.9 as well? There is a bunch of stuff hanging there as well...