diff --git a/Makefile b/Makefile
index 67ddf8c..50d568d 100644
--- a/Makefile
+++ b/Makefile
@@ -49,7 +49,8 @@ CPPFLAGS += -Iinclude \
 	    -D_FILE_OFFSET_BITS=64 \
 	    -DSO_RXQ_OVFL=40 \
 	    -DPF_CAN=29 \
-	    -DAF_CAN=PF_CAN
+	    -DAF_CAN=PF_CAN \
+	    -D_GNU_SOURCE
 
 PROGRAMS_ISOTP = isotpdump isotprecv isotpsend isotpsniffer isotptun isotpserver isotpperf
 PROGRAMS_CANGW = cangw
diff --git a/asc2log.c b/asc2log.c
index ae78a41..81dbdb9 100644
--- a/asc2log.c
+++ b/asc2log.c
@@ -44,7 +44,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/time.h>
-#define __USE_XOPEN /* supress warning for strptime */
 #include <time.h>
 #include <libgen.h>
 #include <unistd.h>
diff --git a/configure.ac b/configure.ac
index df9a193..fdfc300 100644
--- a/configure.ac
+++ b/configure.ac
@@ -88,6 +88,7 @@ AC_CHECK_DECL(AF_CAN,,
 AC_CHECK_DECL(N_SLCAN,,
     [AC_DEFINE([N_SLCAN], [17], [N_SLCAN])]
 )
+AC_DEFINE(_GNU_SOURCE)
 
 
 #
diff --git a/slcanpty.c b/slcanpty.c
index 431ca68..17b8349 100644
--- a/slcanpty.c
+++ b/slcanpty.c
@@ -22,9 +22,6 @@
  *
  */
 
-/* To get ptsname grantpt and unlockpt definitions from stdlib.h */
-#define _GNU_SOURCE
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>