diff --git a/package/libfastjson/libfastjson.mk b/package/libfastjson/libfastjson.mk
index b1cdd6a158..06da0d8edd 100644
--- a/package/libfastjson/libfastjson.mk
+++ b/package/libfastjson/libfastjson.mk
@@ -7,6 +7,7 @@
 LIBFASTJSON_VERSION = v0.99.4
 LIBFASTJSON_SITE = $(call github,rsyslog,libfastjson,$(LIBFASTJSON_VERSION))
 LIBFASTJSON_INSTALL_STAGING = YES
+LIBFASTJSON_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99'
 # From git
 LIBFASTJSON_AUTORECONF = YES
 LIBFASTJSON_LICENSE = MIT