Check for va_copy declaration

ctx.c contains a workaround for systems without va_copy() support.
Improve this workaround in the way which is described in the autoconf
manual.

Also check for va_copy declaration in configure time.

Fixes #160.

configure.ac

@@ -33,6 +33,7 @@ AC_ARG_ENABLE([cares],
 
 AC_SEARCH_LIBS([socket], [network socket])
 AC_CHECK_FUNCS([snprintf vsnprintf])
+AC_CHECK_DECLS([va_copy], [], [], [#include <stdarg.h>])
 
 if test "x$enable_tls" != xno; then
     PKG_CHECK_MODULES([openssl], [openssl],