configure: check for __builtin_available() availability (#1788)

This change does two things:
1. It un-breaks the build in Xcode 9.0. (Xcode 9.0 is currently
   failing trying to compile connectx() in lib/connect.c.)
2. It finally weak-links the connectx() function, and falls back on
   connect() when run on older operating systems.

1 files changed, 19 insertions, 10 deletions

diff --git a/lib/connect.c b/lib/connect.c
index 8d5d32ba6..927ff7cec 100644..100755
--- a/lib/connect.c
+++ b/lib/connect.c
@@ -1059,16 +1059,25 @@ static CURLcode singleipconnect(struct connectdata *conn,
   if(!isconnected && (conn->socktype == SOCK_STREAM)) {
     if(conn->bits.tcp_fastopen) {
 #if defined(CONNECT_DATA_IDEMPOTENT) /* OS X */
-      sa_endpoints_t endpoints;
-      endpoints.sae_srcif = 0;
-      endpoints.sae_srcaddr = NULL;
-      endpoints.sae_srcaddrlen = 0;
-      endpoints.sae_dstaddr = &addr.sa_addr;
-      endpoints.sae_dstaddrlen = addr.addrlen;
-
-      rc = connectx(sockfd, &endpoints, SAE_ASSOCID_ANY,
-                    CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT,
-                    NULL, 0, NULL, NULL);
+#ifdef HAVE_BUILTIN_AVAILABLE
+      if(__builtin_available(macOS 10.11, iOS 9.0, tvOS 9.0, watchOS 2.0, *)) {
+#endif /* HAVE_BUILTIN_AVAILABLE */
+        sa_endpoints_t endpoints;
+        endpoints.sae_srcif = 0;
+        endpoints.sae_srcaddr = NULL;
+        endpoints.sae_srcaddrlen = 0;
+        endpoints.sae_dstaddr = &addr.sa_addr;
+        endpoints.sae_dstaddrlen = addr.addrlen;
+
+        rc = connectx(sockfd, &endpoints, SAE_ASSOCID_ANY,
+                      CONNECT_RESUME_ON_READ_WRITE | CONNECT_DATA_IDEMPOTENT,
+                      NULL, 0, NULL, NULL);
+#ifdef HAVE_BUILTIN_AVAILABLE
+      }
+      else {
+        rc = connect(sockfd, &addr.sa_addr, addr.addrlen);
+      }
+#endif /* HAVE_BUILTIN_AVAILABLE */
 #elif defined(MSG_FASTOPEN) /* Linux */
       if(conn->given->flags & PROTOPT_SSL)
         rc = connect(sockfd, &addr.sa_addr, addr.addrlen);