Make 'tools/build.py -h' warn the user to run it from the repo root.

tools/build.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 import optparse
 import os
 import re
@@ skipping 16 matching lines @@
 
 $ wget http://src.chromium.org/chrome/trunk/src/build/install-build-deps.sh
 OR
 $ svn co http://src.chromium.org/chrome/trunk/src/build; cd build
 Then,
 $ chmod u+x install-build-deps.sh
 $ ./install-build-deps.sh --arm --no-chromeos-fonts
 """
 DEFAULT_ARM_CROSS_COMPILER_PATH = '/usr/bin'
 
+usage = """\
+usage: %%prog [options] [targets]
+
+This script runs 'make' in the *current* directory. So, run it from
+the Dart repo root,
+
+  %s ,
+
+unless you really intend to use a non-default Makefile.""" % DART_ROOT
 
 def BuildOptions():
-  result = optparse.OptionParser()
+  result = optparse.OptionParser(usage=usage)
   result.add_option("-m", "--mode",
       help='Build variants (comma-separated).',
       metavar='[all,debug,release]',
       default='debug')
   result.add_option("-v", "--verbose",
       help='Verbose output.',
       default=False, action="store_true")
   result.add_option("-a", "--arch",
       help='Target architectures (comma-separated).',
       metavar='[all,ia32,x64,simarm,arm,simmips,mips,simarm64,arm64,]',
@@ skipping 489 matching lines @@
           else:
             if BuildOneConfig(options, target, target_os,
                               mode, arch, cross_build) != 0:
               return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())