"make dist" shouldn't create doxygen documentation

[Christian Stimming]

Just recently I discovered that our Makefile rules are set up in a way that 
will include the doxygen HTML output into the "make dist" tarball packages. 
This is true for both HEAD and gnome2-dev CVS and, even worse, also for 
gnucash-1-8-branch.

I propose to remove that rules altogether. The doxygen HTML documentation is 
only interesting for developers who really want to program. Those guys should 
be intelligent enough to follow the directions from the README, which tells 
them to call "make doc" in order to have the doxygen documentation created. 
IMHO there is no reason for shipping a generated version of that HTML with 
the tarball. It only increases building time of "make dist" and increases the 
size of the release tarball. (And for those with historic interest: This rule 
was introduced in the gnucash-1.7.x series, and the ChangeLog says 
"2002-11-27 Doxygen is now required for make dist.")

Unless anyone objects with very important reasons, I would remove those rules 
from src/doc/Makefile.am. Note again that this doesn't affect the manual 
creation of the doxygen documentation by "make doc". It will only turn off 
that step when running "make dist" or "make distcheck".

Christian