Add a test that public headers compile as C99
diff --git a/templates/Makefile.template b/templates/Makefile.template
index 1e9753a..435b407 100644
--- a/templates/Makefile.template
+++ b/templates/Makefile.template
@@ -1858,6 +1858,12 @@
$(LIBDIR)/$(CONFIG)/lib${dep}.a\
% endfor
+ % if tgt.language == 'c89':
+ % for src in tgt.src:
+ $(OBJDIR)/$(CONFIG)/${os.path.splitext(src)[0]}.o: CFLAGS := <%text>$(CFLAGS)</%text> -std=c89
+ % endfor
+ % endif
+
% endfor
% if not has_no_sources:
deps_${tgt.name}: $(${tgt.name.upper()}_OBJS:.o=.dep)