support/script/cpan: improve Makefile generation

- remove trailing space after perl when it's the only dependency
- license: substitution of perl name by BR name
- add a tabulation before source

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>