Add an explicit test for the availablility of 'php' from the command line during setup

Summary: See T481. We'll fail the pcntl test if we don't have this, in a
potentially confusing way. Test and detect missing 'php' explicitly before we
try the pcntl test, so we can give the user a better error message.

Test Plan: In setup mode, did a good run and then faked it to execute 'phpx'
instead to get a failure.

Reviewers: johnduhart, jungejason, tuomaspelkonen, aran

Reviewed By: tuomaspelkonen

CC: aran, epriestley, tuomaspelkonen

Differential Revision: 878