The output directory is not defined by the DOXYGEN_OUTPUT_DIRECTORY
environment variable instead of being hardcoded to `html`. Both the
make rule and the bash script set that variable to `html` by default,
but in the case of the bash script we can override it by defining
the variable in the environement before running the script.
This commit adds the doxygen build framework to the main
scummvm repo.
This allows to easily build and test API doc when making any
kind of changes in doxygen blocks in header files.
The build can be run from command line in the 'doxygen'
folder with 'doxygen scummvm.doxyfile'.
Output is produced in the 'html' folder and can be viewed
by opening the 'index.html' file.
The 'buildfiles' folder contains files related to style.
The output uses the vanila doxygen skin.
