[Bug #21845] Rebuild gem extensions at RUBY_ABI_VERSION change

As `TARGET_SO_DIR_TIMESTAMP` contains `ruby_version`, after bumping
`RUBY_ABI_VERSION` it should not be existing.  Usually such outdated
files will be removed by `make outdate-bundled-gems` automatically
invoked by `make up`.

Author: nobu

ext/extmk.rb

@@ -592,6 +592,7 @@ def create_makefile(*args, &block)
 build_complete = $(TARGET_GEM_DIR)/gem.build_complete
 install-so: build_complete
 clean-so:: clean-build_complete
+$(build_complete) $(OBJS): $(TARGET_SO_DIR_TIMESTAMP)
 
 build_complete: $(build_complete)
 $(build_complete): $(TARGET_SO)