diff --git a/src/libiso8601/build.lib b/src/libiso8601/build.lib
index cde6658..080592f 100644
--- a/src/libiso8601/build.lib
+++ b/src/libiso8601/build.lib
@@ -12,9 +12,9 @@ then
     source src/libiso8601/soversion
 
     libiso8601="obj/${libiso8601_BASE}.so.${SOMAJOR}.${SOMICRO}"
-    libiso8601_DEP_CFLAGS="-std=gnu99"
+    libiso8601_DEP_CFLAGS=""
     libiso8601_DEP_LIBS="-lrt"
-    SO_EXTRA="${libiso8601_DEP_CFLAGS} ${libiso8601_DEP_LIBS} -lc"
+    SO_EXTRA="-std=gnu99 -D_GNU_SOURCE ${libiso8601_DEP_CFLAGS} ${libiso8601_DEP_LIBS} -lc"
 
     echo "Building library ${libiso8601}..."