Coding Rules - eliminate msvc warning C4668 (symbol is not defined as a preprocessor macro, replacing with '0' for directive)

Some headers, like Standard_Macro.hxx, contain preprocessor macros for platform dependent code.
Some of this code like f.e.

#elif (defined(__GNUC__) && __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)) || defined(__clang__)

is not consistent with respect to guard expressions.

Although both, the C and the C++ standard, explicitly allow using undefined identifiers and replace them with '0', see f.e. N3797, 16.1.4 (p. 402)

"After all replacements due to macro expansion and the defined unary operator
have been performed, all remaining identifiers and keywords149, except for true and false, are replaced with the pp-number 0, and then each preprocessing token is converted into a token"

a expression which consistently uses these rules would, using the example of the #elif above, be either

#elif (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6))) || defined(__clang__)

or

#elif (defined(__GNUC__) && __GNUC__ > 4 || (defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ >= 6)) || defined(__clang__)

or

#elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)) || defined(__clang__)

so that either all binary expressions are guarded by a unary one or none of them.

Although this is trivial, it reduces the noise during compilation for compilers where the value use of undefined preprocessor identifiers can be enabled as a warning; f.e. GCC provides the "-Wundef" flag and newer versions of Visual Studio seem to have this enabled per default when using /Wall.