meson: Improve detection of Python when using Meson >=0.50.

meson: Improve detection of Python when using Meson >=0.50.

Previously, on systems where multiple versions of Python 3 (e.g. 3.6 and 3.7)
are installed, wrong version of Python 3 could have been used.

The proper fix requires availability of path() method in Meson's python
module, which has been added in Meson 0.50:
https://github.com/mesonbuild/meson/pull/4616

Distro Bug: https://bugs.gentoo.org/671308
Signed-off-by: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
v2: - Add missing `endif` keyword (Dylan)