Backport #5955: recent Apple Xcode headers need this

reference: https://github.com/arvidn/libtorrent/issues/2364#issuecomment-336175406
(cherry picked from commit e201675a3aa63712f8d08c27f2cedc10873b8f4c)

diff --git a/m4/pdns_check_os.m4 b/m4/pdns_check_os.m4
index ba6546dfac2ea1bcfafefec52f7a289087c882f3..860f9aa0e35029fc897694041e36ab55f4decc57 100644 (file)
--- a/m4/pdns_check_os.m4
+++ b/m4/pdns_check_os.m4
@@ -19,7 +19,7 @@ AC_DEFUN([PDNS_CHECK_OS],[
     have_linux="yes"
     ;;
   darwin*)
-    CXXFLAGS="-D__APPLE_USE_RFC_3542 -D_XOPEN_SOURCE $CXXFLAGS"
+    CXXFLAGS="-D__APPLE_USE_RFC_3542 -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE $CXXFLAGS"
     ;;
   freebsd*)
     THREADFLAGS="-pthread"