BUILD/MINOR: fix solaris build with clang.

clang 9 sets the level to POSIX 2004.

(cherry picked from commit 4831db2f32077469472ebda03e176e6c77dc7472)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4a89a5dab4c60a808e237f35e9e95412d1bd917a)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/Makefile b/Makefile
index 82fa9dd..6a165df 100644
--- a/Makefile
+++ b/Makefile
@@ -380,7 +380,7 @@
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_CRYPT_H USE_GETADDRINFO USE_THREAD \
     USE_RT USE_OBSOLETE_LINKER USE_EVPORTS USE_CLOSEFROM)
-  TARGET_CFLAGS  = -DFD_SETSIZE=65536 -D_REENTRANT -D_XOPEN_SOURCE=500 -D__EXTENSIONS__
+  TARGET_CFLAGS  = -DFD_SETSIZE=65536 -D_REENTRANT -D_XOPEN_SOURCE=600 -D__EXTENSIONS__
   TARGET_LDFLAGS = -lnsl -lsocket
 endif