#31 Add support for --docdir/--htmldir configure flags

[Daniel Pielmeier]

Currently the doc Makefile uses "DOCDIR = $(DESTDIR)$(datadir)/doc/@PACKAGE@" for installing the documentation.
What about using "DOCDIR = $(DESTDIR)$(docdir)" and if unset defaulting that to "$(DESTDIR)$(datadir)/doc/@PACKAGE@"

Considering that only html documentation is installed it is probably a good idea to use "HTMLDIR = $(DESTDIR)$(htmldir)/html"
and default to "HTMLDIR = $(DESTDIR)$(datadir)/doc/@PACKAGE@/html"

This way the installation path can be overridden by the --docdir/--htmldir configure flag.