Fix build of forkfd_qt.cpp on FreeBSD

It no longer compiled after 614f37c8b5 due to
the following:
- forkfd_qt.cpp set _XOPEN_SOURCE to 500
- It then includes qatomic.h which include sys/cdefs.h (the FreeBSD header
  that parses and sets _POSIX_C_SOURCE, _XOPEN_SOURCE and other macros)
- sys/cdefs.h redefines _POSIX_C_SOURCE to 199506 due to _XOPEN_SOURCE's
  value
- Several libc symbols expected to exist by libc++ are hidden due to
  _POSIX_C_SOURCE's value and the build fails

Setting _XOPEN_SOURCE to 700 ensures that _POSIX_C_SOURCE is set to
200809 which is required for libc++ to work correctly

Task-number: QTBUG-45006
Change-Id: Iac93220d19ca5ab9ba8ac61a79748252283c3c47
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/3rdparty/forkfd/forkfd.c

@@ -38,7 +38,7 @@
 #ifndef _GNU_SOURCE
 #  define _GNU_SOURCE
 #  define _POSIX_C_SOURCE 200809L
-#  define _XOPEN_SOURCE 500
+#  define _XOPEN_SOURCE 700
 #endif
 #include "forkfd.h"
 

src/corelib/io/forkfd_qt.cpp

@@ -36,7 +36,7 @@
 #  define _POSIX_C_SOURCE 200809L
 #endif
 #if !defined(_XOPEN_SOURCE) && !defined(__QNXNTO__) && !defined(ANDROID)
-#  define _XOPEN_SOURCE 500
+#  define _XOPEN_SOURCE 700
 #endif
 
 #include <QtCore/qatomic.h>