Building with Ninja instead of Make

[John Wiegley]

I highly recommend the "ninja" build utility over make, if you do frequent
rebuilds of Ledger.  It's *hyper* fast at discovering which dependencies need
to be rebuilt.  And since we're using CMake now, it's trivial to get working:

    cd ~/Products/ledger
    cmake ~/src/ledger -DUSE_DOXYGEN=1 -DUSE_PYTHON=1 -DBUILD_DEBUG=1 \
          -GNinja -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=ON \
          -DCMAKE_CXX_COMPILER:PATH=/usr/local/bin/clang++
    ninja -j16 && ctest -j16

I don't quite have Ninja tied into acprep yet, that's next.

John