Fix forkfd build when O_CLOEXEC isn't defined.

O_CLOEXEC was introduced with the 2008 revision of POSIX.1 and it's the
only way of doing child processes safely with fork(2) in multithreaded
applications.

But we need to support pre-2008 systems, so we can't use that constant.
So let's just choose two arbitrary values for both of our constants --
we need to change both because we need to be sure that FFD_CLOEXEC won't
be the same as FFD_NONBLOCK.

Linux will probably implement them to the O_ constants, like epoll,
signalfd and inotify have done.

Change-Id: I20a5aa6e6264e7a219e19759eeb8747e01df05ff
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>

src/3rdparty/forkfd/forkfd.c

@@ -404,7 +404,7 @@ int forkfd(int flags, pid_t *ppid)
 #endif
     {
         /* try a pipe */
-        if (create_pipe(sync_pipe, O_CLOEXEC) == -1) {
+        if (create_pipe(sync_pipe, FFD_CLOEXEC) == -1) {
             /* failed both at eventfd and pipe; fail and pass errno */
             goto err_close;
         }

src/3rdparty/forkfd/forkfd.h

@@ -44,8 +44,8 @@
 extern "C" {
 #endif
 
-#define FFD_CLOEXEC  O_CLOEXEC
-#define FFD_NONBLOCK O_NONBLOCK
+#define FFD_CLOEXEC  1
+#define FFD_NONBLOCK 2
 
 #define FFD_CHILD_PROCESS (-2)