Parse escaped spaces in makefile dependencies

This means that if a project has a file with a space in the name it will
properly have its freshness calculated as opposed to always having it as a
candidate to be rebuilt.

Closes #648
auto merge of #663 : alexcrichton/cargo/issue-648, r=brson

This means that if a project has a file with a space in the name it will
properly have its freshness calculated as opposed to always having it as a
candidate to be rebuilt.

Closes #648