Update CMakeLists of doxygen project to require at least cmake 3.8.0 (same...

Update CMakeLists of doxygen project to require at least cmake 3.8.0 (same version as viewer) to get rid of deprecation warning.

indra/doxygen/CMakeLists.txt

@@ -4,7 +4,7 @@
 # other commands to guarantee full compatibility
 # with the version specified
 ## prior to 2.8, the add_custom_target commands used in setting the version did not work correctly
-cmake_minimum_required(VERSION 2.8.8 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.8.0 FATAL_ERROR)
 
 set(ROOT_PROJECT_NAME "SecondLife" CACHE STRING
     "The root project/makefile/solution name. Defaults to SecondLife.")