Set the policy to 'OLD' to avoid warning CMP0026 in CMake 3.0.
Merge #954 'Fix the policy warning CMP0026 in CMake 3.0.'
CMake: Generate helptags during install step.

Fix #1056.
Merge #1106 'CMake: Generate helptags during install step.'
CMake: Use DESTDIR for helptags generation.
Merge #1110 'CMake: Use DESTDIR for helptags generation.'