Fix #1601 - prevent threaded execution when pcre.jit is set to 1

src/Psalm/Internal/Fork/Pool.php

@@ -58,6 +58,18 @@ class Pool
             );
         }
 
+        if (ini_get('pcre.jit') === '1'
+            && \PHP_OS === 'Darwin'
+            && version_compare(PHP_VERSION, '7.3.0') >= 0
+        ) {
+            die(
+                'Mac users: pcre.jit is set to 1 in your PHP config.' . PHP_EOL
+                . 'The pcre jit is known to have problems in PHP 7.3 on Macs, and Psalm' . PHP_EOL
+                . 'will not execute in threaded mode to avoid indecipherable errors'
+                . PHP_EOL
+            );
+        }
+
         // We'll keep track of if this is the parent process
         // so that we can tell who will be doing the waiting
         $is_parent = false;