variables in our scons setup.

Here is an example of importing an additional environment variable:

export MYVAR='value'
scons importenv="MYVAR"

Review URL: http://codereview.chromium.org/60046

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1671 ce2b1a6d-e550-0410-aec6-3dcde31c8c00