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

scripts/arcanist.php

@@ -55,7 +55,8 @@ foreach ($args as $key => $arg) {
   }
 }
 
-if (!posix_isatty(STDOUT)) {
+// The POSIX extension is not available by default in some PHP installs.
+if (function_exists('posix_isatty') && !posix_isatty(STDOUT)) {
   PhutilConsoleFormatter::disableANSI(true);
 }