diff --git a/configure.ac b/configure.ac
index 1255db9..f77f7ca 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,6 +26,8 @@ AC_TYPE_SIZE_T
 # Checks for library functions.
 AC_FUNC_MALLOC
 AC_FUNC_REALLOC
+
+AC_CHECK_HEADERS([sys/types.h])
 AC_CHECK_FUNCS([memset letoh32 htole32])
 
 AC_CHECK_HEADERS([endian.h sys/endian.h])
@@ -36,6 +38,7 @@ fi
 AC_CHECK_HEADERS([byteswap.h])
 
 AC_DEFINE([_XOPEN_SOURCE], [600], [Define to 500 if Single Unix conformance is wanted, 600 for sixth revision.])
+AC_DEFINE([_BSD_SOURCE], [1], [Define on BSD to activate all library features])
 
 CFLAGS="$CFLAGS -std=c99"
 
diff --git a/libfreefare/mifare_classic.c b/libfreefare/mifare_classic.c
index 47de8f0..892fdfd 100644
--- a/libfreefare/mifare_classic.c
+++ b/libfreefare/mifare_classic.c
@@ -31,12 +31,15 @@
 
 #include "config.h"
 
+#if defined(HAVE_SYS_TYPES_H)
+#  include <sys/types.h>
+#endif
+
 #if defined(HAVE_SYS_ENDIAN_H)
 #  include <sys/endian.h>
 #endif
 
 #if defined(HAVE_ENDIAN_H)
-#  define _BSD_SOURCE
 #  include <endian.h>
 #endif
 #include <errno.h>