(v)asprintf: define HAVE_(V)ASPRINTF if found

(cherry picked from commit e982db92e1cf61970d7057c7f651c6f07d17d2b2)
Signed-off-by: Rafaël Carré <funman@videolan.org>

configure.ac

@@ -527,14 +527,14 @@ AC_LINK_IFELSE([
         char *c;
         if (asprintf(&c, "%s %d", "string", 1) == -1)
             c = NULL;
-    ])],,[AC_LIBOBJ([asprintf])])
+    ])],[AC_DEFINE([HAVE_ASPRINTF],[1],[Define to 1 if you have asprintf function])],[AC_LIBOBJ([asprintf])])
 AC_LINK_IFELSE([
     AC_LANG_PROGRAM([#include <stdio.h>], [
         char *c;
         va_list ap;
         if (vasprintf(&c, "%s %d", ap) == -1)
             c = NULL;
-    ])],,[AC_LIBOBJ([vasprintf])])
+    ])],[AC_DEFINE([HAVE_VASPRINTF],[1],[Define to 1 if you have asprintf function])],[AC_LIBOBJ([vasprintf])])
 
 # Windows CE does not have strcoll()
 AC_FUNC_STRCOLL