[build-script] Prevent sleeping while building.

I've only implemented this for OS X, which comes with the
'caffeinate' command. If anyone would like to implement
something similar for Linux, feel free. (Although please
make sure that it's signal-safe, i.e. if the build is
interrupted the computer's sleep settings return to normal.)

utils/SwiftBuildSupport.py

@@ -19,6 +19,7 @@ except ImportError:
 
 import os
 import pipes
+import platform
 import subprocess
 import sys
 
@@ -72,7 +73,13 @@ def quote_shell_command(args):
     return " ".join([pipes.quote(a) for a in args])
 
 
-def check_call(args, print_command=False, verbose=False):
+def check_call(args, print_command=False, verbose=False, disable_sleep=False):
+    if disable_sleep:
+        if platform.system() == 'Darwin':
+            # Don't mutate the caller's copy of the arguments.
+            args = list(args)
+            args.insert(0, "caffeinate")
+
     if print_command:
         print(os.getcwd() + "$ " + quote_shell_command(args))
     try: