Autotools fix perl path

LSF-GLIBC package at CERN installs Perl symlink (/bin/perl) pointing
to /usr/bin/perl. All machines by default have Perl on /usr/bin/perl
(incl. Darwin). /bin/perl ends up the first in PATH, thus packages
assume Perl is installed under /bin/perl causing breakage on machines
without LSF-GLIBC.

Always set perl to /usr/bin/perl to avoid issues.