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)