diff --git a/package/ustr/ustr.mk b/package/ustr/ustr.mk
index 3144c0b47c..817f0b0c5e 100644
--- a/package/ustr/ustr.mk
+++ b/package/ustr/ustr.mk
@@ -12,6 +12,8 @@ USTR_LICENSE_FILES = LICENSE LICENSE_BSD LICENSE_LGPL LICENSE_MIT
 
 USTR_INSTALL_STAGING = YES
 
+USTR_CFLAGS = $(TARGET_CFLAGS) -std=gnu89
+
 ifeq ($(BR2_STATIC_LIBS),y)
 USTR_BUILD_TARGETS = all
 USTR_INSTALL_TARGETS = install
@@ -24,7 +26,8 @@ USTR_INSTALL_TARGETS = all install-shared
 endif
 
 define USTR_BUILD_CMDS
-	$(MAKE) -C $(@D) $(TARGET_CONFIGURE_OPTS) $(USTR_BUILD_TARGETS)
+	$(MAKE) -C $(@D) $(TARGET_CONFIGURE_OPTS) CFLAGS="$(USTR_CFLAGS)" \
+		$(USTR_BUILD_TARGETS)
 endef
 
 define USTR_INSTALL_STAGING_CMDS