Add python binary check to build configuration

Currently when a user attempts to build folly without a python binary on
their machine, the build will fail with a long and cryptic error message
from the Makefile.

This commit will add the autoconf macro `m4/ax_python.m4` which will
look for a python executable as well as the associated header files and
libraries. If found, the macro will then set the binary path to
`PYTHON_BIN`. That variable will then be used by the Makefile to execute
the `build/generate*.py` files.

This commit rectifies the following use cases:

Problem:
The user's python binary is followed by the python version number. This
happens in Linux distributions such as Arch Linux and possibly others.
Solution:
This patch will recurse through several python versions looking for a
possible executable followed by its version number and then execute
python scripts using the found executable.

Problem:
The user does not have a python version on their machine and is confused
as to why make fails, presenting a cryptic and verbose error message.
Solution:
This patch will cause the configuration stage of the build to fail while
subsequently presenting a clear error message explaining that python is
required for the build to complete.