dev-lang/coq: Fix -scm version

Upstream switched to git, and the newest versions require camlp5.
Masking dev-lang/coq-scm
dev-lang/coq: Version bump to 8.4pl2

Fixes compilation with Make 4.00
dev-lang/coq: Version pl2 fixup
dev-lang/coq: Include configure patch for make 4
dev-lang/coq: Exlib fixub
Add Exherbo's standard .gitignore
Add Exherbo's standard .gitattributes