python executable

The __os_install_macro defines some post-processing activities during an rpm
build; one of the scripts it calls is brp-python-bytecompile, which can take
an argument: the python executable with which to byte-compile .py files in the
package payload.

In some older versions of rpm (e.g. in RHEL 6), this invocation doesn't pass
in an argument, and brp-python-bytecompile defaults to using /usr/bin/python,
which can lead to the .py files being byte-compiled for the wrong version of
python.  This has been fixed in later versions of rpm by passing in
%{__python} as an argument to brp-python-bytecompile.

Workaround this by detecting if __os_install_post has a 0-argument invocation
of brp-python-bytecompile, and if so generating an equivalent macro that has
the argument, and explicitly provide the new definition within the specfile.