#39 Generate user commands by configure, more autoconf checks

[Joachim Schrod]

(This was reported by Andreas Matthias.)

If one calls configure a second time after make, with different options, the user command xindy is not generated again. It should be, as one could change paths or other stuff.

The root cause is that we use make to produce xindy, instead of configure. This should be changed.

While we're at it, more system dependencies might be inserted by autoconf, e.g., the path of the Perl interpreter. (Quite often, one wants to use the one that is installed in /usr/local/bin/perl and not the one in /usr/bin. But if we do so, also add a --with-perl option to let the user (or the distributor) specify the Perl command that shall be used.)