#18 DoxygenConfig.java: Inject HAVE_DOT=YES, if dot[.exe] exists

[John 'Frotz' Fa'atuai]

RELEASE:
[] 1.6

REQUIREMENT:
[] If dot[.exe] can be found on the PATH, inject
HAVE_DOT=YES into the base configuration.

[] Need to be explicit/verbose about the implicit
environment configuration in the Ant Task output. 1
line max.

USAGE:
[] If the user avoids an explicit reference to
HAVE_DOT, then the automatic detection will work,
while Doxygen will (I think, need to verify this)
assume the parameter to be HAVE_DOT=NO.

[] Users who then have GraphViz on their PATH get
the benefit.

JUSTIFICATION:
[] Doxygen output without GraphViz is very boring
with a borderline value-add.

[] Adding implicit GraphViz detection / enabling,
allows build files to migrate across environments
which do not have GraphViz installed or on the PATH
without breaking the build files. (Graceful
degradation.)