gyp-win-tool: Don't use shell=True on non-Windows hosts.

pylib/gyp/win_tool.py

Index: pylib/gyp/win_tool.py
diff --git a/pylib/gyp/win_tool.py b/pylib/gyp/win_tool.py
index 194bf3da9a74317a08da0e752bfa4a4e3f4c1b1b..1c843a0b6cf528b4888273c5ed0b45c69ffbf987 100755
--- a/pylib/gyp/win_tool.py
+++ b/pylib/gyp/win_tool.py
@@ -119,7 +119,15 @@ class WinTool(object):
     if sys.platform == 'win32':
       args = list(args)  # *args is a tuple by default, which is read-only.
       args[0] = args[0].replace('/', '\\')
-    link = subprocess.Popen(args, shell=True, env=env,
+    # https://docs.python.org/2/library/subprocess.html:
+    # "On Unix with shell=True [...] if args is a sequence, the first item
+    # specifies the command string, and any additional items will be treated as
+    # additional arguments to the shell itself.  That is to say, Popen does the
+    # equivalent of:
+    #   Popen(['/bin/sh', '-c', args[0], args[1], ...])"
+    # For that reason, since going through the shell doesn't seem necessary on
+    # non-Windows don't do that there.
+    link = subprocess.Popen(args, shell=sys.platform == 'win32', env=env,
                             stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
     out, _ = link.communicate()
     for line in out.splitlines():