__VA_OPT__ should be treated like a parameter

[ehaas]

__VA_OPT__ should be stringizable and its parameters should be macro-substituted.

#define FOO BAR
#define M(x, y, ...) #__VA_OPT__(y ## x __VA_ARGS__)
M(a, b, FOO)

Should produce "ba BAR"; currently it errors with error: '#' is not followed by a macro parameter