Tested on Mac OS X 10.9

1.18.0 - works
1.19.0 - fails
1.20.0 - fails
1.21.0 - fails

Failure example:

$ make
emcc -c -o testlib.o testlib.c
emar rcs libtest.a testlib.o
emcc -s ERROR_ON_UNDEFINED_SYMBOLS=1 -L. -ltest -o test.js test.c
error: unresolved symbol: testlibfunc
aborting from js compiler due to exception: Aborting compilation due to previous errors | undefined
Traceback (most recent call last):
  File "/Users/brion/src/emscripten-lib/emscripten/emscripten.py", line 1463, in <module>
    _main(environ=os.environ)
  File "/Users/brion/src/emscripten-lib/emscripten/emscripten.py", line 1451, in _main
    temp_files.run_and_clean(lambda: main(
  File "/Users/brion/src/emscripten-lib/emscripten/tools/tempfiles.py", line 39, in run_and_clean
    return func()
  File "/Users/brion/src/emscripten-lib/emscripten/emscripten.py", line 1459, in <lambda>
    DEBUG_CACHE=DEBUG_CACHE,
  File "/Users/brion/src/emscripten-lib/emscripten/emscripten.py", line 1346, in main
    jcache=jcache, temp_files=temp_files, DEBUG=DEBUG, DEBUG_CACHE=DEBUG_CACHE)
  File "/Users/brion/src/emscripten-lib/emscripten/emscripten.py", line 858, in emscript_fast
    assert '//FORWARDED_DATA:' in out, 'Did not receive forwarded data in pre output - process failed?'
AssertionError: Did not receive forwarded data in pre output - process failed?
Traceback (most recent call last):
  File "/Users/brion/src/emscripten-lib/emscripten/emcc", line 1608, in <module>
    final = shared.Building.emscripten(final, append_ext=False, extra_args=extra_args)
  File "/Users/brion/src/emscripten-lib/emscripten/tools/shared.py", line 1436, in emscripten
    assert os.path.exists(filename + '.o.js'), 'Emscripten failed to generate .js'
AssertionError: Emscripten failed to generate .js
make: *** [test.js] Error 1