Faster NaCl builds using cnacl

This changes the NaCl formula to use cnacl for the NaCl build process.
cnacl is an alternative build system for NaCl which uses cmake for the
build process and supports plans for commonly used architectures, as
opposed to the NaCl library's own build system which performs an
elaborate set of benchmarks to choose the most efficient implementation
for the underlying architecture.

For comparison, here's before:

    $ time brew install nacl
    ==> Downloading http://hyperelliptic.org/nacl/nacl-20110221.tar.bz2
    ########################################################################
    100.0%
    ==> ./do
    ==> ar -r build/horus/lib/amd64/libnacl.a
    build/horus/lib/amd64/randombytes.o
    /usr/local/Cellar/nacl/20110221: 41 files, 564K, built in 12.5 minutes
    brew install nacl  619.48s user 120.07s system 98% cpu 12:27.67 total

And after:

    $ time brew install nacl
    ==> Downloading
    https://github.com/cjdelisle/cnacl/archive/f23fbf8e17771d3623a91db13239f456277ba945.tar.gz.tar.gz
    ########################################################################
    100.0%
    ==> cmake ..
    ==> make
    /usr/local/Cellar/nacl/20110221: 26 files, 416K, built in 6 seconds
    brew install nacl  10.33s user 4.26s system 208% cpu 6.981 total

The build time went from 747.67s using the original formula to 6.981s
with the cnacl-based formula. That's over a **100X speedup**!

Using cmake also dramatically simplifies the formula, which previously
had to work around the library author's's nonstandard build system and
architecture-specific file locations.