configure script should fails when `csh` is not found

[proppy]

without csh currently the build fails with:

../utils/main.h:27:10: fatal error: database/database.h: No such file or directory
   27 | #include "database/database.h"
      |          ^~~~~~~~~~~~~~~~~~~~~

while the configure script seems to have a provision to enforce this dependency here: https://github.com/RTimothyEdwards/magic/blob/master/scripts/configure.in#L285

it doesn't seems to actually fails in practice, because the check expects $CSH to be empty while the configure script set it to no: https://github.com/RTimothyEdwards/magic/blob/master/scripts/configure#L5310 https://github.com/RTimothyEdwards/magic/blob/master/scripts/configure#L5314

[RTimothyEdwards]

I'll take care of it. No need to file a pull request.