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.

Source/cmGlobalUnixMakefileGenerator3.h

@@ -149,6 +149,8 @@ public:
   /** Does the make tool tolerate .DELETE_ON_ERROR? */
   virtual bool AllowDeleteOnError() const { return true; }
 
+  bool IsIPOSupported() const CM_OVERRIDE { return true; }
+
   void ComputeTargetObjectDirectory(cmGeneratorTarget* gt) const CM_OVERRIDE;
 
   std::string IncludeDirective;