Add Doxygen support

* Adds a "make doc" command, outputting documentation in
  $BUILD_DIR/doc

CMakeLists.txt

@@ -59,3 +59,5 @@ add_subdirectory(src)
 
 add_subdirectory(test)
 
+add_subdirectory(doc)
+

doc/CMakeLists.txt

+find_package(Doxygen)
+
+if (DOXYGEN_FOUND)
+    set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile.in)
+    set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}/doc/Doxyfile)
+    configure_file(${DOXYGEN_IN} ${DOXYGEN_OUT} @ONLY)
+    add_custom_target( doc ALL
+        COMMAND ${DOXYGEN_EXECUTABLE} ${DOXYGEN_OUT}
+        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+        COMMENT "Generating documentation with Doxygen"
+        VERBATIM )
+else (DOXYGEN_FOUND)
+    message("Doxygen not found, cannot build documentation")
+endif (DOXYGEN_FOUND)

doc/Doxyfile.in

+PROJECT_NAME     = "ngraph++"
+PROJECT_BRIEF    = "Nervana graph compiler"
+
+OUTPUT_DIRECTORY = @CMAKE_CURRENT_BINARY_DIR@
+INPUT            = @CMAKE_CURRENT_SOURCE_DIR@/../src
+
+RECURSIVE        = YES