cmake ignore doxygen option when not a standalone build (#488)

CMakeLists.txt

@@ -191,13 +191,16 @@ endif()
 #=============================================================================
 # Doxygen
 #
-option(BUILD_DOXYGEN "Build doxygen documentation" OFF)
+# Only in standalone build
+if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+
+	option(BUILD_DOXYGEN "Build doxygen documentation" OFF)
 	
-if (BUILD_DOXYGEN)
+	if (BUILD_DOXYGEN)
 		find_package(Doxygen)
 		if (DOXYGEN_FOUND)
 			# set input and output files
-		set(DOXYGEN_IN ${CMAKE_SOURCE_DIR}/docs/Doxyfile.in)
+			set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/docs/Doxyfile.in)
 			set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
 	
 			# request to configure the file
@@ -216,4 +219,5 @@ if (BUILD_DOXYGEN)
 		else()
 			message(FATAL_ERROR "Doxygen needs to be installed to generate documentation")
 		endif()
+	endif()
 endif()