diff --git a/configure.in b/configure.in
index 69f5a44..b1755bf 100644
--- a/configure.in
+++ b/configure.in
@@ -27,6 +27,10 @@ dnl Needed on some glibc systems
 CFLAGS="$CFLAGS -D_BSD_SOURCE"
 CFLAGS="$CFLAGS -D_ISO9X_SOURCE"
 
+dnl Needed on Digital Unix
+CFLAGS="$CFLAGS -D_OSF_SOURCE"
+CFLAGS="$CFLAGS -D_XOPEN_SOURCE_EXTENDED"
+
 AC_C_CONST
 AC_C_INLINE