uwsgiconfig: cleanup plugin cflags blacklist
uwsgiconfig: move comment where it is supposed to be
uwsgiconfig: cleanup external plugin cflags / ldflags blacklist
uwsgiconfig: cleanup plugin configuration a bit

Unify embedded and external plugins configuration so that we don't
duplicate code and bugs. While at it fix remote plugin compilation
when the repo uri ends with .git (c&p from github).

Tested compilation with plain make, external git plugin (uwsgi-realtime)
and internal plugin with --plugin switch.
Merge pull request #763 from xrmx/configcleanup

Cleanup plugin build machinery