check on doc/CMakeLists since is already checked on main CMakeLists.
At same time, Doxygen becomes required as no documentation can be built
without it.
Output directory now uses the user defined HTML_OUTPUT instead of
forced dir.
- Added included GNUInstallDirs on same UNIX check, avoiding duplication
of checks
Current documentation lacks a proper directory handling and switch for
Unix like systems.
The option BUILD_DOCS are added as disable by default, even so the
Doxyfile file is generated for a manual build.
Option HTML_OUTPUT are made cached to be properly replaced, as usually
done by some Linux distributions
Microsoft CHM option is enabled if MSVC is detected.
Helio Chissini de Castro