Provide pipe2() and mkostemp() fallbacks for Darwin.

Darwin has no pipe2() at all; mkostemp() appeared in macOS 10.12; O_CLOEXEC
appeared in the 10.7 SDK. Emulate the exact subset the Wayland backend uses
(O_CLOEXEC / O_NONBLOCK via fcntl). The private O_CLOEXEC value is only
tested against by the shims themselves and never reaches the kernel.

--- src/video/wayland/SDL_waylanddatamanager.c
+++ src/video/wayland/SDL_waylanddatamanager.c
@@ -37,6 +37,43 @@
 #include "SDL_waylanddatamanager.h"
 #include "primary-selection-unstable-v1-client-protocol.h"
 
+#ifdef SDL_PLATFORM_APPLE
+/* Darwin has no pipe2(), and SDKs before Mac OS X 10.7 lack O_CLOEXEC. The
+   flags are only interpreted here and are never passed to the kernel. */
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0x1000000
+#endif
+static int Wayland_pipe2(int fds[2], int flags)
+{
+    int i;
+
+    if (pipe(fds) == -1) {
+        return -1;
+    }
+    for (i = 0; i < 2; ++i) {
+        if (flags & O_CLOEXEC) {
+            if (fcntl(fds[i], F_SETFD, FD_CLOEXEC) == -1) {
+                goto error;
+            }
+        }
+        if (flags & O_NONBLOCK) {
+            const int fl = fcntl(fds[i], F_GETFL);
+            if (fl == -1 || fcntl(fds[i], F_SETFL, fl | O_NONBLOCK) == -1) {
+                goto error;
+            }
+        }
+    }
+    return 0;
+
+error:
+    close(fds[0]);
+    close(fds[1]);
+    fds[0] = fds[1] = -1;
+    return -1;
+}
+#define pipe2 Wayland_pipe2
+#endif // SDL_PLATFORM_APPLE
+
 /* This is arbitrary, but reading while polling should block for less than a frame, to
  * prevent hanging while pumping events.
  *
--- src/video/wayland/SDL_waylandshmbuffer.c
+++ src/video/wayland/SDL_waylandshmbuffer.c
@@ -33,6 +33,29 @@
 #include "SDL_waylandshmbuffer.h"
 #include "SDL_waylandvideo.h"
 
+#ifdef SDL_PLATFORM_APPLE
+/* Darwin gained mkostemp() only in macOS 10.12, and SDKs before Mac OS X
+   10.7 lack O_CLOEXEC. The flag is only interpreted here and is never
+   passed to the kernel. */
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0x1000000
+#endif
+static int Wayland_mkostemp(char *tmpl, int flags)
+{
+    const int fd = mkstemp(tmpl);
+
+    if (fd >= 0 && (flags & O_CLOEXEC)) {
+        if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+            close(fd);
+            unlink(tmpl);
+            return -1;
+        }
+    }
+    return fd;
+}
+#define mkostemp Wayland_mkostemp
+#endif // SDL_PLATFORM_APPLE
+
 static bool SetTempFileSize(int fd, off_t size)
 {
 #ifdef HAVE_POSIX_FALLOCATE