Fix make apps from fresh clone
Express dependency between core and bc targets

```
$ git grep '\.\./' | grep '^[A-z]\+\/[A-z]\+:'
...
core/Makefile:117:include/bif.h: ../bc/scripts/bif.tab
core/Makefile:129:gentab/exp_tab.erl: $(PREMODS) ../bc/scripts/bif.tab
core/Makefile:130:      scripts/exptab_gen preload ../bc/scripts/bif.tab $@
...
```
Merge pull request #12 from lucafavatella/fix-make

Fix make from fresh clone
Add build icon
Merge pull request #13 from lucafavatella/core-depends-on-bc

Express dependency between core and bc targets
Fix icon link
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