Add evpp port file

[zieckey]

This adds support for evpp, a modern C++ network library based on libevent for developing high performance network services in TCP/UDP/HTTP protocols (https://github.com/Qihoo360/evpp).

[msftclas]

@zieckey,
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[msftclas]

@zieckey, thanks for signing the contribution license agreement. We will now validate the agreement and then the pull request.

Thanks, Microsoft Pull Request Bot

[ras0219-msft]

Thanks for the PR!

Some suggested improvements:

• Using vcpkg_from_github() would enable you to pull from the latest master via install --head evpp, which might be able to replace your EVPP_LOCAL_TEST.
• Each of the vcpkg helpers prints their own statuses, so the extra messages in the portfile are probably unnecessary.

[zieckey]

Thanks. Got it.
We will improve it for next version.