pfff: fix race in find_php_files

Summary:
Pieter discovered this bug. I also had this problem
sometimes with codegraph and never really understood why ...
until now. Thx Pieter.

Test Plan: make test

Reviewers: pieter, jiaol

Reviewed By: pieter

Differential Revision: https://phabricator.fb.com/D900661
* lang_php/parsing/lib_parsing_php.ml:
* env.sh: misc
* external/ocamlpcre/: removing
CEs on removal of ocamlpcre in Makefiles
Merge branch 'master' of github.com:facebook/pfff