Stop checking for duplicate basenames

Chromium doesn't care about MSVC08 or Mac OS X makefiles anymore,
whereas some third_party libraries we'd like to use within Chromium
contain source files with duplicate basenames.

Discussion link: https://groups.google.com/a/chromium.org/d/msg/chromium-dev/0rDiXgLvywY/sm0LSj-hiDIJ

Patch from mdempsky@chromium.org

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/670063006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1993 78cadc50-ecff-11dd-a971-7dbc132099af