Introduce '--no-duplicate-basename-check' option to disable the check of duplicate basenames

With this CL, the check of duplicate basenames in the same source list can be disabled with '--no-duplicate-basename-check' option.

Now GYP generators can handle duplicate basenames except for VCProj generator for Visual C++ 2008 and Makefile generator on Mac. Given that these two generators are no longer actively used, and supposed to be deprecated in future, providing an option to disable this validation should be helpful for some GYP users.

Note that these two generators continue to treat duplicate basenames continue as an error regardless of '--no-duplicate-basename-check'.

BUG=gyp:264, gyp:384
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/344573002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1947 78cadc50-ecff-11dd-a971-7dbc132099af