game: Log succeses in game_shader_from_source()
ply: Add submodule

Add submodule at path "external/ply".

    git://github.com/chadversary/ply-turk
    refs/heads/master
cmake/ply: Build external project ply

Build source at "external/ply" and install it to the prefix
${CMAKE_BINARY_DIR}. This adds the following phony targets:
    external-ply
    external-ply-configure
    external-ply-build
    external-ply-test
    external-ply-install

Import libply as the CMake target "ply".
game: Remove ply.[ch] and instead use external/ply
cmake: Simplify external project target names

Rename:
    external-${project}            -> ${project}
    external-${project}-${command} -> ${project}->${command}

This caused a conflict with the target names of imported library. So
rename each imported library target as

    ${library} -> ${library}_imported