configure: require python mako module

configure: require python mako module

It is now a hard dependency because of the autogeneration of
format pack and unpack functions.

Update the documentation to reflect this change.

v2:
- Inline python script in m4 file and use PYTHON2

v3:

- Remove semicolons and quotes and change coding style
- Add Ilia Mirkin suggestion to use Python's split functionality.
- Use AX_CHECK_PYTHON_MAKO_MODULE name.
- Change to MIT license

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>