Refactor cmake (#1231)

Fixes #920.

cmake/Docs.cmake

@@ -0,0 +1,20 @@
+find_package(Doxygen REQUIRED)
+
+# See Doxyfile for these settings:
+set(SOURCE ${CMAKE_CURRENT_SOURCE_DIR}/..)
+set(USE_DOT "YES")
+set(LINT "NO")
+set(EXCLUDES "")
+# ---
+
+set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile)
+set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
+
+configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT})
+add_custom_target(
+  docs
+  COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+  COMMENT "Generating API documentation with Doxygen"
+  VERBATIM
+)