Introduce {v,}asprintf for windows

We have now again two functions called h_platform_vasprintf and
h_platform_asprintf. On windows they are implemented in terms of vsnprint
and the like. On BSD/GNU libraries we use the supplied vasprintf and
asprintf.

src/platform.h

@@ -8,8 +8,17 @@
 
 #include "compiler_specifics.h"
 
+#include <stdarg.h>
 #include <stdint.h>
 
+/* String Formatting */
+
+/** see GNU C asprintf */
+int h_platform_asprintf(char **strp, const char *fmt, ...);
+
+/** see GNU C vasprintf */
+int h_platform_vasprintf(char **strp, const char *fmt, va_list arg);
+
 /* Error Reporting */
 
 /* BSD errx function, seen in err.h */