Don't copy the ENV variable from the users's external environment into...

Don't copy the ENV variable from the users's external environment into
construction environments.  SCons uses this as a dictionary of environment
values for the commands it executes.
BUG=none
TEST=build with the variable ENV set in the user environment

[sgk, 2009-08-24 20:42:59]

Mark Mentovai ran into this; he sets a variable named ENV in his environment,
and the import of all user variables into the construction environment
conflicted with SCons' use.

[Mads Ager (chromium), 2009-08-25 06:29:05]

Thanks for the patch, I will land it.

http://codereview.chromium.org/173294/diff/1/2
File SConstruct (right):

http://codereview.chromium.org/173294/diff/1/2#newcode806
Line 806: sample_flags = context.AddRelevantFlags(os.environ, SAMPLE_FLAGS)
Should use |user_environ| here as well.