UseCorrade.cmake: more cleanup.
UseCorrade.cmake: make corrade_add{,_static}_plugin() nearly the same.

It is now possible to conveniently install static plugins too using the
same syntax. It allows to do for example this:

    macro(add_plugin)
        if(NOT BUILD_STATIC)
            corrade_add_plugin(${ARGN})
        else()
            corrade_add_static_plugin(${ARGN})
        endif()
    endmacro()
Utility: don't send empty messages through NaClConsoleStreamBuffer.