sub have_functions {
    my @funcs = ();
    print "detecting available functions... ";

    my @paths = DynaLoader::dl_findfile(qw(-lpcap));
    my $libref = DynaLoader::dl_load_file($paths[0]);

    for my $func (@_) {
        my $symref = DynaLoader::dl_find_symbol($libref, $func);
        push @funcs, $func if defined $symref
    }

    print "ok\n";
    return @funcs
}

...

# Check that '-lwpcap' exports 'pcap_open_live'
print have_functions('pcap_open_live');