cmake: add doxygen

CMakeLists.txt

@@ -1,7 +1,9 @@
-cmake_minimum_required(VERSION 3.8 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
 
 project(olive-editor LANGUAGES CXX)
 
+option(BUILD_DOXYGEN "Build Doxygen documentation" OFF)
+
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
@@ -77,6 +79,8 @@ else()
   message("Olive: No git hash defined!")
 endif()
 
+find_package(Doxygen)
+
 set(OLIVE_SOURCES
   dialogs/aboutdialog.cpp
   dialogs/aboutdialog.h
@@ -462,6 +466,14 @@ if(WIN32 AND OPENCOLORIO_FOUND)
   target_link_libraries(${OLIVE_TARGET} PRIVATE OpenColorIO)
 endif()
 
+if(DOXYGEN_FOUND AND BUILD_DOXYGEN)
+  set(DOXYGEN_PROJECT_NAME "Olive")
+  set(DOXYGEN_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/docs")
+  set(DOXYGEN_EXTRACT_ALL "YES")
+  set(DOXYGEN_EXTRACT_PRIVATE "YES")
+  doxygen_add_docs(docs ALL ${OLIVE_SOURCES})
+endif()
+
 if(UNIX AND NOT APPLE)
   install(TARGETS ${OLIVE_TARGET} RUNTIME DESTINATION bin)
   install(FILES ${OLIVE_EFFECTS} DESTINATION share/olive-editor/effects)