Added untar of ogre tagfile

doc/CMakeLists.txt

@@ -1,7 +1,7 @@
 find_package(Doxygen)
 
 set(IGNITION_DOXYGEN_TAGFILES
-    "\"${CMAKE_SOURCE_DIR}/doc/ogre_1-9.tag.xml.tgz=https://www.ogre3d.org/docs/api/1.9/\"")
+  "\"${CMAKE_BINARY_DIR}/doc/ogre_1-9.tag.xml=https://www.ogre3d.org/docs/api/1.9/\"")
 
 if (DOXYGEN_FOUND)
   configure_file(${IGNITION_CMAKE_DOXYGEN_DIR}/api.in
@@ -10,13 +10,24 @@ if (DOXYGEN_FOUND)
   configure_file(${IGNITION_CMAKE_DOXYGEN_DIR}/tutorials.in
                  ${CMAKE_BINARY_DIR}/tutorials.dox @ONLY)
 
-  add_custom_target(doc ALL
-    # Generate the API documentation
-    ${DOXYGEN_EXECUTABLE} ${CMAKE_BINARY_DIR}/api.dox
-    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+  # Find tar
+  find_program(TAR_PATH tar)
+  set (DOC_DEPEND "")
 
-    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_BINARY_DIR}/tutorials.dox
+  # If tar exists, then untar the ogre xml tag file
+  if (TAR_PATH)
+    add_custom_target(doc_xml ALL
+      ${TAR_PATH} xvf ${CMAKE_SOURCE_DIR}/doc/ogre_1-9.tag.xml.tgz -C ${CMAKE_BINARY_DIR}/doc
+      WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
+    set (DOC_DEPEND doc_xml)
+  endif()
 
+  # Generate the API and tutorial documentation
+  add_custom_target(doc ALL
+    ${DOXYGEN_EXECUTABLE} ${CMAKE_BINARY_DIR}/tutorials.dox
+    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+    COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_BINARY_DIR}/api.dox
+    DEPENDS ${DOC_DEPEND}
     COMMENT "Generating API documentation with Doxygen" VERBATIM)
 
   install(FILES ${CMAKE_BINARY_DIR}/doc/${PROJECT_NAME_LOWER}.tag.xml