Standardized Makefile for man page

Standardized Makefile for man page
Use our standard build machinery for building, installing and cleaning the
man page. Do not use deprecated -<num> option for 'head'. Use $(RM) instead
of explicit 'rm -f' for better portability.

Note that we always build/install the man page with the 'install-docs' target
now, this is the usual behaviour. "GhcManpages is dead, Jim..."

This patch should unbreak 'make dist', too.