configure.ac: Do not clobber LIBS when looking for libperl

We're using ExtUtils::Embed to figure out the libraries needed for the
perl module, do not clobber the global LIBS when verifying that libperl
exists. Previously, everything got linked against libperl, which is
undesired.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>