Use GNUInstallDirs for documentation.
Use GNUInstallDirs for documentation.

The directory will be exactly the same[1] by default, but using a
variable makes it easier for packagers and other people who need to
install it in a different directory.

[1] <https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html>

mobius3/tweeny