test: add windows-only snprintf() function

Should fix the build after 96f32a2 inadvertently broke it.

There is no snprintf() on Windows because, hey, it's a C99 addition
and the people from Redmond, WA are still firmly stuck in 1989.
build: add automake serial-tests version check

The serial-tests directive was added in automake v0.11. Add an ad-hoc
version check to find out if it's safe to enable. Fixes the autotools
build with older versions of automake.