gplugin/gplugin

Don't bother looking for libperl on FreeBSD as it's there if the perl executable is installed.

Testing Done:
Tested on debian with and without libperl-dev installed as well as FreeBSD with perl installed.

Reviewed at https://reviews.imfreedom.org/r/246/
gplugin_query = executable('gplugin-query',
'gplugin-query.c',
dependencies : [gplugin_dep, GLIB, GOBJECT],
install : get_option('install-gplugin-query'))
if get_option('help2man')
custom_target('gplugin-query.1',
command : [help2man,
'--name=Query installed plugins', '--section=1',
'--help-option=--help-all', '--no-info',
'--output', '@OUTPUT@',
gplugin_query],
output : 'gplugin-query.1',
install : get_option('install-gplugin-query'),
install_dir : get_option('mandir') / 'man1')
endif