The bit that goes at the start of Makefile.PL (or Build.PL) looks something like this:

use lib qw(inc);
use Devel::CheckLib;

# Prompt the user here for any paths and other configuration

check_lib_or_exit(
    # fill in what you prompted the user for here
    lib => [qw(cthulhu fhtagn)]
);
[download]

While you could, in theory, extract all the relevant code from the module and put it straight into your Makefile.PL, there's a *lot* of it. Building and linking C code in a cross-platform way is a pain in the arse.