CMakeLists.txt: Add LANGUAGES option to project variable

[bkuhls]

Fixes build with toolchains without c++.

[madeye]

Thanks. The intent — restrict the project to C so no C++ compiler is required — is good, but set(PROJECT_NAME shadowsocks-libev LANGUAGES C) makes PROJECT_NAME the list shadowsocks-libev;LANGUAGES;C, which only works by accident when later expanded into project(). Please set the languages directly on the project() call instead. Note master's cmake_minimum_required was just bumped to 3.10 (#3034), so a rebase is needed.