Fix 'implicit declaration of vsnprintf' GCC warning (if strict ANSI mode)

* cord/tests/cordtest.c (GC_SNPRINTF): Redirect to sprintf also in
case of __STRICT_ANSI__ (i.e., if -ansi compiler option specified).
* misc.c (GC_VSNPRINTF): Redirect to vsprintf also in case of
__STRICT_ANSI__.