Autotools version failed to configure on macOS, and QrEncode's README says "If
the configure script does not work well, try to use CMake." Cmake works for
both macOS and Windows.
Allows future macOS configuration in each individual build script to not all be
duplicated. Also deduplicates current Windows arg checking and config setting.
foo=dirname "$0" only works if the script is called with an absolute path and
that path contains no spaces. Add quotes everywhere to contain spaces in
directory names or file names, and add realpath to get the absolute path
regardless of call path.
Not necessarily needed, but like in e405868037
it can avoid some annoying issues when e.g. that script was called with a
relative path in CI.
