BLD: some updates to Bento config files and docs.  Closes gh-3978.

Update MACOSX_DEPLOYMENT_TARGET to 10.6 (same as most python.org installers
use) and the version to 0.16.0dev0.

Also add the correct SourceForge download URL and update the build instructions
to use the right Waf version.

[ci skip]
Merge pull request #4228 from rgommers/fix-issue-3978

BLD: some updates to Bento config files and docs.  Closes gh-3978.