How to require proper <thread> header for port build?

[Lev Serebryakov]

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 My new port requires C++0x, and I've added "USES=compiler:c++0x" to
port's Makefile. But on 9.x it could not be built, as this USES
doesn't guarantee presence of "#include <thread>" — port is built with
clang++, but there is no such header.

 What should I specify in port to support building on 9.x in such case?

- -- 
// Lev Serebryakov
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