Old Z3 is not supported (should enforce in CMake)

I have z3 4.8.13 installed.

CMake properly warns (I think for the reason that different versions may produce different outputs for the tests):
```
CMake Error at CMakeLists.txt:74 (message):
SMTChecker tests require Z3 4.8.17 for all tests to pass.

Build with -DSTRICT_Z3_VERSION=OFF if you want to use a different version.
You can also use -DUSE_Z3=OFF to build without Z3. In both cases use
--no-smt when running tests.
```

However disabling it (using `-DSTRICT_Z3_VERSION=OFF`) shows that 4.8.13 is not a supported version anymore:
```
/Projects/solidity/libsmtutil/Z3Interface.cpp:381:11: error: use of undeclared identifier 'Z3_OP_RECURSIVE'
kind == Z3_OP_RECURSIVE
^
```

Should add another version check for "not older than".

Start in CMakeLists.txt around line 74 and inspect the existing STRICT_Z3_VERSION check, then read the Z3 version-dependent code in libsolidity/smtutil/Z3Interface.cpp around line 381. Make the configuration check reject Z3 4.8.13 and other unsupported older versions before compilation, while preserving the documented behavior for STRICT_Z3_VERSION=OFF.

Written by the indexing model from the issue text.