Allow doxygen parameters to be passed to task
Allow doxygen parameters to be passed to task

The doxygen demo program suggests that doxy parameters can be passed
as arguments to the task, e.g.,

   bld(features="doxygen", doxyfile="doxy.conf",
        pars={'INPUT': bld.path.ant_glob("**/*.c")})

Previously, these inputs were not passed from the task generator
to the task, and only the parameters read from the doxy configuration
file were used. This patch retrieves parameters (if any) from the task
generator and uses them to override those in the doxy file,
allowing more flexible overriding for individual build targets.

Signed-off-by: Thomas Nagy <d86e025e60d31fa2452cf443d0fed051ea678233@gmail.com>

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