LIB_ONLY issue when building libshadowsocks-libev.a by CMake

It seems that building libshadowsocks-libev.a by CMake with no LIB_ONLY defined. It makes no _start_ss_local_server when some EXE linking with libshadowsocks-libev.a. Locally I resolved it by updating src/CMakeLists.txt:184 to:

target_compile_definitions(shadowsocks-libev PUBLIC -DMODULE_LOCAL -DLIB_ONLY)

[madeye]

Please open a pull request.

@madeye Sorry, I can't. My environment is a mess now.