When trying to get the version of a program, meson was previously
hardcoded to run the binary with `--version`. This does work with the
vast majority of programs, but there are a few outliers (e.g. ffmpeg)
which have an unusual argument for printing out the version. Support
these programs by introducing a version_argument kwarg in find_program
which allows users to override `--version` with whatever the custom
argument for printing the version may be for the program.