R1:d3558eb25fc0

Explicitly detect missing pcntl and exit with an obvious error

Summary: We already detect this in setup but double-check just in case, see
gmail thread.
Test Plan: Rebuilt PHP without pcntl and tried to run phd, got an error.
Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen
CC: aran
Differential Revision: 819