Building Doxygen Documentation

Ensure the following:

1. In the root of your vapor repository open CMakeLists.txt. Ensure the build-doxygen option is ON. option (BUILD_DOC "Build Vapor Doxygen documentation" ON)

2. In your build directory, open CMakeCache.txt. Ensure that the BUILD_DOC option is ON BUILD_DOC:BOOL=ON

3. In your build directory, issue the make doc command.