Improve the way vim looks for python binaries

Previously, we patched in the names of all the python binary versions, but
I have been doing a terrible job of keeping them up-to-date. Instead, just
tell vim's configure script the path to the binary we're using.

This will fix building against python 3.7.

PR: 230675
Reported by: Kamigishi Rei
MFH: 2018Q3