Squashed 'CMakeModules/' changes from e83ac80..ebfa49d

ebfa49d Merge pull request #1 from arpg/private_linkage
40245bc Makes def_library link privately.
2979d55 Fix install when version string is empty.

git-subtree-dir: CMakeModules
git-subtree-split: ebfa49d8c26ef31a783425596febca3f457820ec
Merge commit 'ee232b2101f6a5f3c5699f2716254a7786b97ebb'
Merge pull request #3 from arpg/private_linkage

Updates def_library.