Allow to create a static binary for Linux using -DBUILD_STATIC_BIN=ON

This patch to CMake allows to create a static binary easily. Just do:
```
$ cmake -DBUILD_STATIC_BIN=ON .. 
$ make
```

Both me and @TrevorHansen had some trouble creating a static binary for STP in SMT competition. I eventually had to resort to manually fixing the linker command. I think it should be much easier to do this. This patch fixes it for me at least. It should probably be tested with other configs and other systems (win32? Mac?)