dev-lang/php: add "coverage" USE flag to build with --enable-gcov.

dev-lang/php: add "coverage" USE flag to build with --enable-gcov.

We add three new revisions, one for each supported branch of PHP. The
new revisions have a "coverage" flag, which was chosen because a few
other packages in ::gentoo have decided on the same flag name. When
enabled, USE=coverage will pull in dev-util/lcov from the Linux Test
Project, and PHP will be built with --enable-gcov.

Since users won't be running code coverage reports on the PHP source
tree itself (at least not the one used by their package manager), this
is probably only useful for extension developers who would like to run
code coverage reports on the test suites for their extensions. It also
requires GCC to work, since lcov wraps GCC's gcov. Both of those
caveats have been mentioned in the USE flag description.

Gentoo-Bug: 542178

Package-Manager: portage-2.3.0