Clover and scrutinizer targets for code coverage.

The scrutinizer target downloadas the uploader tool, executes the
`coverage-clover` target and executes the uploader with the code
coverage report.

HHVM does not work with the phar provided by Scrutinizer.
Add option to use different Makefile.

With this option, it is now easy to test different Makefile (from Forks,
for instance) using an *environment variable* and
--environment-overrides option of `make`.

The Makefile URL environment variable used to hold it is
**FOUNDATION_URL**.