clang packager/win: Check that GnuWin32 is in the path at the start.

tools/clang/scripts/package.py

Index: tools/clang/scripts/package.py
diff --git a/tools/clang/scripts/package.py b/tools/clang/scripts/package.py
index 74fc409ebcc3092b5b3191b789dd1e4c25781658..be8498f527e123da01ccffcf952ea5472d8e1a9e 100755
--- a/tools/clang/scripts/package.py
+++ b/tools/clang/scripts/package.py
@@ -58,6 +58,13 @@ def PrintTarProgress(tarinfo):
 
 
 def main():
+  if sys.platform == 'win32':
+    try:
+      subprocess.check_output(['grep', '--help'], shell=True)
+    except subprocess.CalledProcessError:
+      print 'Add gnuwin32 to your PATH, then try again.'
+      return 1
+
   parser = argparse.ArgumentParser(description='build and package clang')
   parser.add_argument('--gcc-toolchain',
                       help="the prefix for the GCC version used for building. "