CMake fails to find doxygen

[jakelangham]

Describe the bug

On the latest commit CMake configuration fails on my local machine with the following error

CMake Error at CMakeLists.txt:233 (doxygen_add_docs):
  Unknown CMake command "doxygen_add_docs".

I am using CMake 3.7.2. Looking at the documentation, it seems that doxygen_add_docs was added in version 3.9. Adding FindDoxygen.cmake from the CMake git repo to cmake/Modules fixes this for me.