Make 'arc' work for users without the POSIX PHP installed.
Make 'arc' work for users without the POSIX PHP installed.

Summary:
Some PHP installs don't have this by default. Do a conditional check
since this functionality is noncritical.

Test Plan:
will make srash test

Differential Revision: 43
Reviewed By: aran
Reviewers: rash67, aran, jungejason
CC: aran

calblueprint/arcanist,coursera/arcanist,kevinseelbach/arcanist,wharris/arcanist,Khan/arcanist,aik099/arcanist,hach-que/arcanist,wxstars/arcanist,Symplicity/arcanist,codelabs-ch/arcanist,Symplicity/arcanist,julkiewicz/arcanist,r4nt/arcanist,eSpark/arcanist,dguo-coursera/arcanist,haskell-infra/arcanist,phacility/arcanist,haskell-infra/arcanist,wharris/arcanist,folsom-labs/arcanist,r4nt/arcanist,zhihu/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,chundongwang/arcanist,sneakpeeq/arcanist,kevinseelbach/arcanist,uber/arcanist,uber/arcanist,haskell-infra/arcanist,hach-que/arcanist,zhihu/arcanist,phacility/arcanist,calblueprint/arcanist,wxstars/arcanist,eSpark/arcanist,PageUpPeopleOrg/arcanist-patches,memsql/arcanist,sneakpeeq/arcanist,coursera/arcanist,wikimedia/phabricator-arcanist,wharris/arcanist,aik099/arcanist,sneakpeeq/arcanist,folsom-labs/arcanist,jra3/arcanist,r4nt/arcanist,memsql/arcanist,KieranHayes/arcanist,alexmv/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,julkiewicz/arcanist,uber/arcanist,chundongwang/arcanist,dguo-coursera/arcanist,codelabs-ch/arcanist,fivestars/arcanist,wxstars/arcanist,kevinseelbach/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,Khan/arcanist,Symplicity/arcanist,sneakpeeq/arcanist,zhihu/arcanist,phacility/arcanist,jra3/arcanist,phacility/arcanist,alexmv/arcanist,calblueprint/arcanist,wxstars/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,kevinseelbach/arcanist,eSpark/arcanist,calblueprint/arcanist,KieranHayes/arcanist,r4nt/arcanist,Khan/arcanist,jra3/arcanist,fivestars/arcanist,memsql/arcanist,eSpark/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,codelabs-ch/arcanist,alexmv/arcanist,alexmv/arcanist,Khan/arcanist,julkiewicz/arcanist,hach-que/arcanist,dguo-coursera/arcanist,uber/arcanist,aik099/arcanist,wikimedia/phabricator-arcanist,zhihu/arcanist,fivestars/arcanist,julkiewicz/arcanist,coursera/arcanist,fivestars/arcanist,KieranHayes/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,memsql/arcanist,haskell-infra/arcanist,Symplicity/arcanist,KieranHayes/arcanist,hach-que/arcanist,chundongwang/arcanist,coursera/arcanist,aik099/arcanist,wharris/arcanist,codelabs-ch/arcanist