Build test suite which is largely C89 with -std=gnu89.

Submitted upstream: <https://github.com/cc65/cc65/issues/2277>

diff --git a/test/ref/Makefile b/test/ref/Makefile
index 5f0b86164d7145d7..42ca263efecbb832 100644
--- a/test/ref/Makefile
+++ b/test/ref/Makefile
@@ -38,7 +38,7 @@ OPTIONS = g O Os Osi Osir Osr Oi Oir Or
 DIFF = $(WORKDIR)$Sbdiff$(EXE)
 
 CC = gcc
-CFLAGS = -O2 -Wall -W -Wextra -funsigned-char -fwrapv -fno-strict-overflow
+CFLAGS = -std=gnu89 -O2 -Wall -W -Wextra -funsigned-char -fwrapv -fno-strict-overflow
 
 .PHONY: all clean