Avoid using MSVC-internal _STRINGIZE

[StephanTLavavej]

I work on Microsoft's C++ Standard Library implementation, where we recently merged microsoft/STL#4405 to remove our internal _STRINGIZE macro. Our "Real World Code" test suite, which builds popular open-source projects like yours, found that you were using this MSVC-internal macro and therefore our change broke your code.

The C++ Standard's rule is that _Leading_underscore_capital identifiers (including _LEADING_UNDERSCORE_ALL_CAPS) are reserved for the compiler and Standard Library, so other libraries and applications should avoid using such reserved identifiers. This is N4971 5.10 [lex.name]/3:

In addition, some identifiers appearing as a token or preprocessing-token are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.
— Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use.

Fortunately, your usage was very limited and you already had a fallback for non-MSVC platforms, so it's simple to rename the affected macros.

Related PR: 3rdparty/stout#72