HxCPP Add Include Paths and Linker Options

Question

I'm trying to use an external C++ library in Haxe. I have an extern class with an @:include meta, but I can't figure out how to add the directory containing some headers to the HxCPP include path, or add linker options to add to the lbrary path and link with the libraries. I assume it will involve using @:buildXml, and I have this:

@:buildXml("
<target id=\"default\">
    <cppflag value=\"-I...\" />
    <flag value=\"-L...\" />
    <lib base="..." />
</target>
")

, but those flags don't show up in the logged g++ command. I can't find any documentation about adding include paths, or any real examples of build XML.