This makes the depot_tools manpages automatically show up in the effective
MANPATH for mac/linux.

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/259863004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@266667 0039d316-1c4b-4281-b951-d872f2087c98