[BioC] Rgraphviz update for BioC 1.9

[Kasper Daniel Hansen]

Rgraphviz has been updated in the release branch of Bioconductor 1.9.  
The new version 1.12.3 replaces 1.12.1 (1.12.2 was visible 1-2 days  
in the repository).

The update makes it possible to use Rgraphviz with Graphviz version  
2.10 and 2.12 (as well as the odd numbered development versions 2.9  
and 2.11). There is no added functionality nor bug fixes so current  
users of Rgraphviz do not need to update their version.

Some added comments on (R)Graphviz:

* It is known that on certain platforms Graphviz 2.8 (and 2.4, 2.6)  
has problems laying out a graph containing singleton nodes with the  
neato algorithm, such as the first graph in the vignette. This  
problem results in a segmentation fault and subsequent crash of the R  
session. The easiest fix is to update Graphviz.

* When installing Graphviz be aware that I have seen problems on the  
Mac OS X Power PC platform when Graphviz is compiled with  
optimization. This problem may or may not persist across platforms.

Kasper