cppcheck: Add new --enable option argument and --library argument completion

It's not very clear from the --library option description what arguments
are expected. It turns out that it expects xml file with .cfg extension.
Some example configs: https://github.com/danmar/cppcheck/tree/master/cfg/
export, _variables: Do TZ= completion (Redhat: #994646).

Could do this for env, declare, and typeset as well, especially
when/if we have more known env vars implemented in _variables.
Cosmetics.
make: Use only posix basic regexps with sed (Alioth: #314345)
_longopt: Run commands in C locale.

This is a follow-up to commit 822735146f
aptitude: safe-upgrade accepts package name as parameters (Alioth: #313638, Debian: 673235)

Patch-by: Nicolas Le Cam <niko.lecam@gmail.com>