Support GCC Variadic Macros for MinGW

[tomrittervg]

MinGW uses the GCC version of Variadic Macros, which is currently the else case. Add a first case for MinGW.

See https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html

[natbro]

thanks - good fix - we'll fold this into the next drop & close it then after you confirm.