Race condition causes jom to hang and/or crash on trivial makefile

A trivial makefile can cause jom to hang or crash when stress-testing it. Most of the time, jom runs as expected but there is a risk it will die. (In real-world usage, this caused the jom generator of CMake to fail intermittently during the configure stage, where jom is repeatedly invoked to test various compiler capabilities).

To reproduce:

1. Save the pre-compiled jom 1.0.15 binary downloaded from http://download.qt.io/official_releases/jom/ someplace on your computer. Put it in your PATH.
2. Save attached Makefile and stress.cmd Windows batch file to some directory on your computer.
3. Run stress.cmd. Eventually it will hang & die / jom will crash. (No error is printed to console - it just silently hangs and/or crashes).
4. If one instance of stress.cmd doesn't do the trick, run several concurrent instances of stress.cmd to load your machine. On my system, at least one jom instance will die within a matter of seconds.