The libjsonparser.so link rule omits $(CFLAGS), so when CFLAGS enables LTO
(e.g. -flto=thin), json.o is compiled as an LLVM bitcode object but linked
without any LTO flag. mold then refuses it:

  mold: fatal: json.o: unable to handle this LTO object file because the
  -plugin option was not provided. Please make sure you added -flto not
  only when creating object files but also when linking the final
  executable.

Pass $(CFLAGS) to the link command too, matching the compile command.

--- a/Makefile.in
+++ b/Makefile.in
@@ -31,7 +31,7 @@
 	$(AR) rcs libjsonparser.a $^

 libjsonparser.so: $(OBJS)
-	$(CC) -shared -Wl,-soname,$(SO_NAME) $(LDFLAGS) -o libjsonparser.so $^ -lm
+	$(CC) $(CFLAGS) -shared -Wl,-soname,$(SO_NAME) $(LDFLAGS) -o libjsonparser.so $^ -lm

 libjsonparser.dylib: $(OBJS)
 	$(CC) -dynamiclib $^ -o libjsonparser.dylib