- fixed: format specifiers like PRIu32 or PRIdMAX are defined in
  standard C header `inttypes.h`

- fixed: `_snprintf` is a function introduced by MicroSoft, which isn't
  fully compliant with `snprintf` from C99 (it doesn't support `%zu`,
  for example). `snprintf`/`vsnprintf` are available in MinGW though

- fixed: `_GNU_SOURCE` was added in `fadb26aa`, Aug 23 2017 in attempt
  to enable declaration of GNU extension `vasprintf`. But later in
  `a06b1b4d` and `6bdd8c0a` more correct way of handling this function
  was added - via definition of subsitution if it's absent (e.g. for
  Solaris); so enforcing `_GNU_SOURCE` isn't necessary anymore, and it
  breaks MinGW build.