Emscripten Install broken by doing a pthreads build.

Version 2.0.6, Windows 10, MinGW shell environment.

I tried building a project with pthreads and now my entire emscripten install is not working at all. I usually get errors like this:

```
em++ -Isrc/ -Wall -Wformat -Os -s USE_SDL=2 -s WASM=1 -s ALLOW_MEMORY_GROWTH=1 -s DISABLE_EXCEPTION_CATCHING=1 -s NO_EXIT_RUNTIME=0 -s ASSERTIONS=1 -c -o build/src/main.cpp.o src/main.cpp
cache:INFO: generating port: libSDL2.a... (this will be cached in "C:\\emsdk\upstream\emscripten\cache\wasm\libSDL2.a" for subsequent builds)
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\pool.py", line 121, in worker
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\pool.py", line 44, in mapstar
File "C:\\emsdk\upstream\emscripten\tools\system_libs.py", line 80, in run_one_command
shared.run_process(cmd, stdout=stdout, stderr=stderr, env=safe_env)
File "C:\\emsdk\upstream\emscripten\tools\shared.py", line 205, in run_process
ret = subprocess.run(cmd, check=check, input=input, *args, **kw)
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\subprocess.py", line 472, in run
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\subprocess.py", line 775, in __init__
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\subprocess.py", line 1178, in _execute_child
FileNotFoundError: [WinError 2] The system cannot find the file specified
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "C:\\emsdk\upstream\emscripten\\em++.py", line 14, in
sys.exit(emcc.run(sys.argv))
File "C:\\emsdk\upstream\emscripten\emcc.py", line 2006, in run
compile_source_file(i, input_file)
File "C:\\emsdk\upstream\emscripten\emcc.py", line 1985, in compile_source_file
cmd = get_clang_command(input_file)
File "C:\\emsdk\upstream\emscripten\emcc.py", line 1930, in get_clang_command
return system_libs.process_args(cmd, shared.Settings)
File "C:\\emsdk\upstream\emscripten\tools\system_libs.py", line 1968, in process_args
port.get(Ports, settings, shared)
File "C:\\emsdk\upstream\emscripten\tools\ports\sdl2.py", line 85, in get
return [shared.Cache.get(libname, create, what='port')]
File "C:\\emsdk\upstream\emscripten\tools\cache.py", line 126, in get
temp = creator()
File "C:\\emsdk\upstream\emscripten\tools\ports\sdl2.py", line 80, in create
ports.run_commands(commands)
File "C:\\emsdk\upstream\emscripten\tools\system_libs.py", line 1710, in run_commands
run_build_commands([add_args(c) for c in commands])
File "C:\\emsdk\upstream\emscripten\tools\system_libs.py", line 94, in run_build_commands
pool.map_async(run_one_command, commands, chunksize=1).get(999999)
File "D:\obj\windows-release\37amd64_Release\msi_python\zip_amd64\pool.py", line 657, in get
FileNotFoundError: [WinError 2] The system cannot find the file specified
```

I have tried `emcc --clear-cache`, but don't know what to do next. This error tells me nothing.