Don't checkout a git branch if not given

"master" is not always the default branch. Just leave it as the default one
that it is cloned from.

Fixes conda/conda#738
Merge branch 'master' into rpath
Add py34 selector variable
Simplify some logic
Extend BuildRoot to track non-DLL files.

(I'm using "DLL" in this context to refer to anything that passes the `is_obj`
test (for Linux, anything that looks like an ELF binary).)
Merge branch 'master' into rpath