Take substring of TOOL_VERSION_OUTPUT, not FIND_TOOL_VERSION_OUTPUT.

Fixes out-of-range error when attempting to locate header to version
information in a version string because FIND_TOOL_VERSION_OUTPUT
has no content to it
Merge pull request #3 from smspillaz/tooling-find-package-cmake-util.item_3

Take substring of TOOL_VERSION_OUTPUT, not FIND_TOOL_VERSION_OUTPUT.