diff --git a/configure.ac b/configure.ac
index 7130152..7199d7c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -498,10 +498,7 @@ AC_CHECK_FUNCS(setutxent utmpxname)
 AC_CHECK_FUNCS(logout updwtmp logwtmp)
 
 # POSIX monotonic time
-OLDCFLAGS="$CFLAGS"
-CFLAGS="$CFLAGS -D_POSIX_C_SOURCE=199309L"
 AC_CHECK_FUNCS(clock_gettime)
-CFLAGS="$OLDCFLAGS"
 
 # OS X monotonic time
 AC_CHECK_HEADERS([mach/mach_time.h])
diff --git a/includes.h b/includes.h
index 0f12620..246882b 100644
--- a/includes.h
+++ b/includes.h
@@ -29,11 +29,6 @@
 #include "options.h"
 #include "debug.h"
 
-#if __linux__
-/* For clock_gettime */
-#define _POSIX_C_SOURCE 199309L
-#endif
-
 #include <sys/types.h>
 #include <sys/ioctl.h>
 #include <sys/param.h> /* required for BSD4_4 define */