Log message:
Add buildlink3.mk file, setting BUILDLINK_DEPMETHOD.rapidxml?=  build
as it installs static headers.

Log message:
Import rapidxml-1.13 as wip/rapidxml.

RapidXml is an attempt to create the fastest XML parser possible, while
retaining usability, portability and reasonable W3C compatibility. It is an
in-situ parser written in modern C++, with parsing speed approaching that of
strlen function executed on the same data.