configure: make call to readlink portable to non-GNU (BSD) systems

The argument '-e' for readlink is a GNU extension.
In setting the variable 'where' the argument '-e' is not necessary, thus eliminated.

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/dokuwiki/projects/crosstool"

 reporter_comment=""