autoconf, fall back to default datadir install location

don't initialise 'with_lispdir' unless the 'compute-lispdir' script is successful. also, give the user a hint on why the default 'datarootdir' based target changed.

i hit this bug as my .emacs sets my local sitepath, hence the 'compute-lispdir' script drew a blank, and so the install-lispLISP target installed the files to ..themselves?! appending my local load path to EMACSLOADPATH worked round the issue but this fix may help others so..