[GNC] Building v3 - Wiki entry for Ubuntu

[Colin Law]

The wiki entry for building v3 on Ubuntu [1] has get me up and running
with little difficulty so many thanks who contributed to writing it.

There is an error there, though, I think.  The section on
install-target says for /usr/local:

/usr/local - this location is commonly used for versions distributed
with linux distributions and is the default option. If installed here
the program will be available to all users on the system. If you use
this folder, use it with caution as installing from the distribution
will overwrite your build and vice versa.

I do not believe that is correct.  The distribution will not generally
install into /usr/local and in fact, at least on Ubuntu, this is a
common place to put home built stuff.  Applications installed there
will not get overwritten by applications installed using apt-get.

Shall I go ahead and change that?

Also I think the title could be improved to indicate that it is for
anything after Ubuntu 16.04. I know it says that in the text but the
title is confusing I think.

Colin

[1] https://wiki.gnucash.org/wiki/BuildUbuntu16.04