Use OpenSSL 1.0 to build PHP 5.6

Summary:
As PHP 5.6.40 has some issues with OpenSSL 1.1,
this change allows to build it against OpenSSL 1.0.

It uses the legacy OpenSSL installation available
per 4f5ee490be79 at /opt/openssl-legacy.

Test Plan: Tested on Ysul, with also D2086.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2087