build: fix inability to detect correct python command in configure
The "which" utility is not guaranteed to be installed, and if it is, its behavior is not portable.
Conversely, the "command -v" shell builtin is required to exist in all POSIX 2008 compliant shells, and is thus guaranteed to work everywhere.
Examples of open-source shells likely to be installed as /bin/sh on Linux, which implement the 12-year-old standard: ash, bash, busybox, dash, ksh, mksh and zsh.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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