Using compile cache can break static library builds on mac

[tobil4sk]

When compiling a Build.xml like this, the build fails on mac while linking the default target with HXCPP_COMPILE_CACHE enabled:

<xml>
	<files id="lib">
		<cache value="true" asLibrary="true" />
		<file name="lib.c" />
	</files>

	<target id="default" output="lib${DBG}" tool="linker" toolid="static_link">
		<files id="lib" />
	</target>
</xml>

fatal error: /Applications/Xcode_15.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file list file: obj/darwin64/all_objs is empty

The problem is that the lib files are cached by hxcpp in a static library, and apart from that there are no object files. This means there is an empty all_objs file generated, which libtool doesn't seem to like. On other platforms (with different static linking tools) this is not an issue.