to avoid errors like:

```
g++ -lssl -lcrypto -o sigtool main.o hash.o macho.o signature.o commands.o
/usr/bin/ld: hash.o: in function `SigTool::SHA256Hash::SHA256Hash(unsigned char const*, unsigned long)':
hash.cpp:(.text+0xcc): undefined reference to `SHA256'
collect2: error: ld returned 1 exit status
make: *** [Makefile:18: sigtool] Error 1
```

The link line is updated to be something like:

```
g++  -o sigtool main.o hash.o macho.o signature.o commands.o -lssl -lcrypto
```