Fix locating Python interpreter when it isn't "python" (mono/mono#18197)

Fix locating Python interpreter when it isn't "python" (mono/mono#18197)

Fix locating Python interpreter when it isn't "python"

The "python" interpreter is specified by PEP-394. In it, an OS
can decide not to ship a "python" executable, and instead only
ship python2/3, providing some ambiguity for scripts that are
version-neutral, like the one Mono uses.

This uses an automake macro to locate a suitable Python, and
changes the makefile to use the located Python, whatever it may be.

Commit migrated from https://github.com/mono/mono/commit/84221f04b5ff1f5f6de69100494ae7cb3ba94975