Make warnings more informative.

CMakeLists.txt

@@ -403,9 +403,12 @@ if(OPENMW_LTO_BUILD)
             set(CMAKE_INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE)
         else()
             message(WARNING "Requested option OPENMW_LTO_BUILD not supported by this compiler: ${HAVE_IPO_OUTPUT}")
+            if(MSVC)
+                message(STATUS "Note: Flags used to be set manually for this setting with MSVC. We now rely on CMake for this. Upgrade CMake to at least 3.13 to re-enable this setting.")
+            endif()
         endif()
     else()
-        message(WARNING "Requested option OPENMW_LTO_BUILD not supported by this cmake version: ${CMAKE_VERSION}")
+        message(WARNING "Requested option OPENMW_LTO_BUILD not supported by this cmake version: ${CMAKE_VERSION}. Upgrade CMake to at least 3.9 to enable support for certain compilers. Newer CMake versions support more compilers.")
     endif()
 endif()