Could not find Doxygen (missing: DOXYGEN_EXECUTABLE) in Mac OS X

Doxygen isn't installed by default. You can download it using brew.

$ brew info doxygen
doxygen: stable 1.8.9.1 (bottled), HEAD
http://www.doxygen.org/
/usr/local/Cellar/doxygen/1.8.9.1 (5 files, 15M) *
  Poured from bottle
From:      https://github.com/Homebrew/homebrew/blob/master/Library/Formula/doxygen.rb
==> Dependencies
Optional: graphviz ✘
==> Options
--with-doxywizard
    Build GUI frontend with qt support.
--with-graphviz
    Build with dot command support from Graphviz.
--with-libclang
    Build with libclang support.
--HEAD
    Install HEAD version

$brew install doxygen