Revert "Merge pull request #1381 from xzfc/master"

This reverts commit 6c0a596dacd7672e650847f1ed15e6a2a67b1483, reversing
changes made to 33d3a7c83b2f1d2cda22b53bf82d68267154cdfd.
CMake: Fix helptags generation.
Merge pull request #1387 from fwalch/fix-installation

Fix installation.