build: fix cross-compilation of python bindings

build: fix cross-compilation of python bindings

AX_PYTHON_DEVEL macro from the autoconf-archive collection doesn't work
when we're trying to cross-compile python bindings in environments such
as buildroot or yocto. It can't properly detect the location of python
headers.

Let's use AM_PATH_PYTHON to detect the python3 interpreter and then
either take the PYTHON_CPPFLAGS and PYTHON_LIBS variables from the
environment (so that cross-compiling build systems can pass their
custom locations to autotools) or try to detect them ourselves using
the python3-config tool.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>