cmake: work with CMake < 2.8.5

CMake versions 2.8.4 and earlier were giving this error since r146323:
"string end index: -1 is out of range 0 - 6"

Passing -1 as the length of the desired substring was a new feature
added in CMake 2.8.5:
http://www.cmake.org/Bug/view.php?id=10740

llvm-svn: 146372