Because AX_PYTHON_DEVEL now provides PYTHON_LIBS instead of PYTHON_FLAGS,
this script needs to be updated accordingly.
This lets AX_BOOST_PYTHON actually find the boost python library
on newer systems that ignore the -l flags if they appear before the
source file on the command line.

I'm not sure why I needed to add PYTHON_LIBS to LIBS in the AC_CACHE_CHECK()
line, though it apparently wasn't necessary to use PYTHON_LDFLAGS there
before. I suspect that much of this file doesn't really make sense.

I have submitted this patch upstream:
https://savannah.gnu.org/patch/index.php?8776