Make gai_strerror optional via configuration

Check via usual autoconf and when not present print just the error
code instead of the readable string. Useful for compact embedded
stacks where this is not present.

Author: fedepell

configure.ac

@@ -105,7 +105,7 @@ AC_CHECK_DECLS([__CYGWIN__])
 AC_SEARCH_LIBS(accept, network socket)
 
 # Checks for library functions.
-AC_CHECK_FUNCS([accept4 getaddrinfo gettimeofday inet_pton inet_ntop select socket strerror strlcpy])
+AC_CHECK_FUNCS([accept4 gai_strerror getaddrinfo gettimeofday inet_pton inet_ntop select socket strerror strlcpy])
 
 # Required for MinGW with GCC v4.8.1 on Win7
 AC_DEFINE(WINVER, 0x0501, _)

src/modbus-tcp.c

@@ -403,7 +403,11 @@ static int _modbus_tcp_pi_connect(modbus_t *ctx)
     rc = getaddrinfo(ctx_tcp_pi->node, ctx_tcp_pi->service, &ai_hints, &ai_list);
     if (rc != 0) {
         if (ctx->debug) {
+#ifdef HAVE_GAI_STRERROR
             fprintf(stderr, "Error returned by getaddrinfo: %s\n", gai_strerror(rc));
+#else
+            fprintf(stderr, "Error returned by getaddrinfo: %d\n", rc);
+#endif
         }
         freeaddrinfo(ai_list);
         errno = ECONNREFUSED;
@@ -629,7 +633,11 @@ int modbus_tcp_pi_listen(modbus_t *ctx, int nb_connection)
     rc = getaddrinfo(node, service, &ai_hints, &ai_list);
     if (rc != 0) {
         if (ctx->debug) {
+#ifdef HAVE_GAI_STRERROR
             fprintf(stderr, "Error returned by getaddrinfo: %s\n", gai_strerror(rc));
+#else
+            fprintf(stderr, "Error returned by getaddrinfo: %d\n", rc);
+#endif
         }
         freeaddrinfo(ai_list);
         errno = ECONNREFUSED;