Building Z3 for StarExec.

TODO: better to use Docker or something?

0. Install cmake.
1. Download Z3 4.8.17 source code (either of the last two in the assets list) from
   https://github.com/Z3Prover/z3/releases/tag/z3-4.8.17 .
2. Extract the archive, and

```
   $ cd z3-4.8.17
   $ mkdir build
   $ cd build
   $ cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXE_LINKER_FLAGS:STRING='-static -Wl,--whole-archive -lrt -lpthread -Wl,--no-whole-archive'
   $ make -j4 # compilation takes long time
```

3. Check if `z3` is produced and statically linked:

```
$ file z3
z3: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=95991503c5806fda5744cbbbd78f8320cc126917, for GNU/Linux 3.2.0, not stripped
```
