Added g++ dependency

On initial compilation, I get an error message that is solved by installing g++:

```
make[1]: Entering directory `/home/amos/urbit/outside/re2'
g++ -o obj/util/arena.o  -Wall -O3 -g -pthread  -Wno-sign-compare -c -I.    -DNDEBUG util/arena.cc
make[1]: g++: Command not found
make[1]: *** [obj/util/arena.o] Error 127
make[1]: Leaving directory `/home/amos/urbit/outside/re2'
make: *** [outside/re2/obj/libre2.a] Error 2
```