Express deps of `railing`

See `escriptize`.
Use local version of nettle at compilation

Sample error:
```
gcc -Wall -Werror -std=gnu99 -O3 -fexcess-precision=standard -frounding-math -mfpmath=sse -msse2 -Wno-nonnull -Wno-strict-aliasing -fno-stack-protector -U_FORTIFY_SOURCE -fno-omit-frame-pointer -flto -D_ISOC99_SOURCE -D_GNU_SOURCE -iquote lib -isystem lib/misc/include -isystem lib/pcre -iquote include -iquote arch/x86/include -iquote bignum -iquote lib/lwip/src/include -iquote lib/lwip/src/include/ipv4 -iquote lib/lwip/src/include/ipv6 -iquote lib/lwip/ling -D__XEN_INTERFACE_VERSION__=0x00030205 -DLING_VER=0.3.1 -o bif_binary.o -c bif_binary.c
In file included from bif_binary.c:38:0:
include/bif_impl.h:43:24: fatal error: nettle/md5.h: No such file or directory
compilation terminated.
make[1]: *** [bif_binary.o] Error 1
make[1]: Leaving directory `/home/travis/build/lucafavatella/ling/core'
make: *** [core] Error 2

The command "make" exited with 2.
```

Snippet taken from
https://github.com/cloudozer/ling/pull/14/files#diff-ce79deecfd2f89db4ccdd711965c38e6R27
Use local nettle dep thus do not install package on TravisCI
Merge pull request #16 from lucafavatella/local-nettle-compile

Use local version of nettle at compilation
Merge pull request #17 from lucafavatella/local-nettle-ci

Use local nettle dep thus do not install package on TravisCI
Merge pull request #15 from lucafavatella/railing-depends-on-bc-and-core

Express deps of `railing`
Bring the future to the Mac.

Tune makefiles to benefit from a cross-compiling GCC toolchain.

- $(CROSS) config variable is introduced
- make railing accept $RAILING_CROSS_PREFIX environment variable
- while here, fix readme markup