Bug#986248: pdfsandwich: detect native-archs for ocaml

[Yangfl]

Source: pdfsandwich

Hi,

pdfsandwich ftbfs on many archs (
https://buildd.debian.org/status/package.php?p=pdfsandwich ) because
of missing ocamlopt. Please consider adjusting makefile according to
https://bugs.debian.org/883018 , thanks.