wmax and wmin C funs could be turned into single macro

[jangorecki]

not a big fan of macros but couldn't we do something like and deduplicate wmin and wmax?

#define DEFINE_WINDOWFUN(fname, init_val, op)
DEFINE_WINDOWFUN(wmax, R_NegInf, >=)
DEFINE_WINDOWFUN(wmin, R_PosInf, <=)

Originally posted by @ben-schwen in #7299 (comment)