Adding CryptoMiniSat4 support

This adds support to check if CryptoMiniSat4 is installed, and if so, build it as part of STP. If CMS4 is installed, the --cryptominisat4 option is added to the binary. Also, the C interface will now be able to handle CMS4 (otherwise, if CMS4 is asked for through the C interface, old CMS is used).

Comments are welcome!