Unused variable in Z3Interface.cpp causes error-as-warning error when compiling with MSVC

[androlo]

On @axic 's request.

Solidity: Version: 0.4.18-develop.2017.10.18+commit.fda8499c.Windows.msvc (after fixing)

C:\Users\andreas\solidity\libsolidity\formal\Z3Interface.cpp(101): error C2220: warning treated as error - no 'object' fil e generated [C:\Users\andreas\solidity\build\libsolidity\solidity.vcxproj]
C:\Users\andreas\solidity\libsolidity\formal\Z3Interface.cpp(101): warning C4101: '_e': unreferenced local variable [C:\Us ers\andreas\solidity\build\libsolidity\solidity.vcxproj]

The cause is an unused error in a catch clause. This only happens because I manually linked with Z3, so it should not affect other users or CI, unless it is if left as-is when Z3 is included in windows builds by default (maybe it already is).

My suggestion was to just disable the warning in the cmake script (/wd4101).