[CMake] Comaptibility with older gcc

[Robert J. Hansen]

> C++ code is not compatible between different compilers.

This is only kinda-sorta true.  Clang and G++ interoperate quite nicely
on Linux, for instance, since they both implement the Itanium-style ABI.
 I believe Intel's C++ compiler on Windows implements the same ABI as
MSVC++.

There are no guarantees of compatibility between different compilers,
but it's a little much to say there is no compatibility.