Configure script now accounts for non-interactive shells

The project configuration requires a build-directory at configuration time,
two other directories can optionally be given to avoid downloading the
project's necessary data and software. It is possible to give these three
directories as command-line options, or by interactively giving them after
running the configure script.

Until now, when these directories weren't given as command-line options,
and the running shell was non-interactive, the configure script would crash
on the line trying to interactively read the user's given directories (the
'read' command).

With this commit, all the 'read' commands for these three directories are
now put within an 'if' statement. Therefore, when 'read' fails (the shell
is non-interactive), instead of a quiet crash, a descriptive message is
printed, telling the user that cause of the problem, and suggesting a fix.

This bug was found by Michael R. Crusoe.

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