+opam-installext.1.0.0

This OPAM plugin will use the `depexts` metadata in the OPAM database to
install any prerequisite system libraries before installing the OPAM package.
It detects your operating system distribution and uses the relevant package
manager.  Supported ones include:

- Linux
  - Debian (`apt-get`)
  - Ubuntu (`apt-get`)
  - CentOS (`yum`)
- BSD
  - OpenBSD (`pkg_add`)
  - FreeBSD (`pkg install`)
  - NetBSD (`pkg_add`)
- Mac OS X
  - Homebrew (`brew`)
  - MacPorts (`port`)
Merge pull request #3296 from avsm/installext

+opam-installext.1.0.0