Introduce variable for where cap’n’proto is installed

Set the `capnp_prefix` variable when calling ./configure.

E.g. build and install cap’n’proto in $HOME/build:

    ./configure --disable-shared --prefix="$HOME/build"
    make -j6 check && make install

Then configure TextMate to find it there:

    capnp_prefix="$HOME/build" ./configure