set std=c89 we cannot set it together with -Wsystem-headers since the system headers are usually not c89 but c99
diff --git a/makefile_include.mk b/makefile_include.mk
index c5cfc28..161321a 100644
--- a/makefile_include.mk
+++ b/makefile_include.mk
@@ -56,7 +56,7 @@ CFLAGS += -Wstrict-prototypes -Wpointer-arith
endif
ifdef CONV_WARNINGS
-CFLAGS += -Wconversion -Wsign-conversion
+CFLAGS += -std=c89 -Wconversion -Wsign-conversion
else
CFLAGS += -Wsystem-headers
endif