Makefile: set MANDIR according to GNU Install Dirs guidelines

[AndersonTorres]

As described in GNU Coding Standards.

This is especially useful for

• software distributions that do not follow FHS
• splitted packages
• slim environments, e.g containers

References:

• https://www.gnu.org/prep/standards/html_node/Makefile-Conventions.html#Makefile-Conventions
• https://www.gnu.org/prep/standards/html_node/Directory-Variables.html#Directory-Variables
• https://refspecs.linuxfoundation.org/fhs.shtml

[rfjakob]

Thanks!