CMake improvements/fixes for old versions

[jacobkahn]

Fixes https://github.com/kpu/kenlm/issues/336.

• Fix CMake going back to 3.1 -- (https://cmake.org/Bug/view.php?id=14444)
  • Moves install calls back to the directories where their librari targets are created
• Make non-transitive dependencies private (compression libs, etc) rather than using generator expressions
• Install auxiliary libraries (besides kenlm/kenlm_util)
• Gate compression lib deps in kenlmConfig by whether they were found at compile time