Allow to build vs nacl instead of libsodium

By default libsodium is used. Only if --enable-nacl is specified, then
nacl will be used instead of libsodium.

Pass locations of nacl headers and libraries by using the following
options:

--with-nacl-headers=/home/me/somewhere/nacl-20110221/build/469/include/amd64/
--with-nacl-libs=/home/me/somewhere/nacl-20110221/build/469/lib/amd64/