Add policy CMP0069 to enforce INTERPROCEDURAL_OPTIMIZATION

Previously the `INTERPROCEDURAL_OPTIMIZATION` target property was
honored only for the Intel compiler on Linux and otherwise ignored.  In
order to add support for more compilers incrementally without changing
behavior in the future, add a new policy whose NEW behavior enforces the
`INTERPROCEDURAL_OPTIMIZATION` property.  Add flags for supported
compilers and otherwise produce an error.

Modules/Platform/Linux-Intel.cmake

@@ -39,6 +39,7 @@ macro(__linux_compiler_intel lang)
       "${XIAR} cr <TARGET> <LINK_FLAGS> <OBJECTS> "
       "${XIAR} -s <TARGET> ")
     set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER YES)
+    set(_CMAKE_IPO_LEGACY_BEHAVIOR YES)
   else()
     set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER NO)
   endif()