Add a Meson option "portable" to choose between "portable" and unix-like
directories. Add information about this option in the README.
To determine the user's directory use the variable USERPROFILE only on
Windows and use HOME otherwise.
Implement the "portable" option in the package build script.
On macOS, we want to use `mkstemp` rather than `tmpnam`. Enable
POSIX support in Lua to fix that (and some other things). Since
POSIX support also enables the flag for POPEN, we no longer need
to do that directly for Linux.
Francesco Abbate
rxi
Bruce Mitchener