Add mechanism to override OS (for testing purposes) (#993)

Similar to `EMSDK_ARCH` which I added in #935.

emsdk.py

@@ -58,10 +58,6 @@ extra_release_tag = None
 VERBOSE = int(os.getenv('EMSDK_VERBOSE', '0'))
 TTY_OUTPUT = not os.getenv('EMSDK_NOTTY', not sys.stdout.isatty())
 
-WINDOWS = False
-if os.name == 'nt' or (os.getenv('SYSTEMROOT') is not None and 'windows' in os.getenv('SYSTEMROOT').lower()) or (os.getenv('COMSPEC') is not None and 'windows' in os.getenv('COMSPEC').lower()):
-  WINDOWS = True
-
 
 def errlog(msg):
   print(msg, file=sys.stderr)
@@ -72,9 +68,27 @@ def exit_with_error(msg):
   sys.exit(1)
 
 
+WINDOWS = False
 MINGW = False
 MSYS = False
-if os.getenv('MSYSTEM'):
+MACOS = False
+LINUX = False
+
+if 'EMSDK_OS' in os.environ:
+  EMSDK_OS = os.environ['EMSDK_OS']
+  if EMSDK_OS == 'windows':
+    WINDOWS = True
+  elif EMSDK_OS == 'linux':
+    LINUX = True
+  elif EMSDK_OS == 'macos':
+    MACOS = True
+  else:
+    assert False
+else:
+  if os.name == 'nt' or (os.getenv('SYSTEMROOT') is not None and 'windows' in os.getenv('SYSTEMROOT').lower()) or (os.getenv('COMSPEC') is not None and 'windows' in os.getenv('COMSPEC').lower()):
+    WINDOWS = True
+
+  if os.getenv('MSYSTEM'):
     MSYS = True
     # Some functions like os.path.normpath() exhibit different behavior between
     # different versions of Python, so we need to distinguish between the MinGW
@@ -85,12 +99,10 @@ if os.getenv('MSYSTEM'):
       # https://stackoverflow.com/questions/37460073/msys-vs-mingw-internal-environment-variables
       errlog('Warning: MSYSTEM environment variable is present, and is set to "' + os.getenv('MSYSTEM') + '". This shell has not been tested with emsdk and may not work.')
 
-MACOS = False
+  if platform.mac_ver()[0] != '':
-if platform.mac_ver()[0] != '':
     MACOS = True
 
-LINUX = False
+  if not MACOS and (platform.system() == 'Linux'):
-if not MACOS and (platform.system() == 'Linux'):
     LINUX = True
 
 UNIX = (MACOS or LINUX)