Corrected two errors connected to 64-bit clauses.

Bit-shifts must work with the correct type, and the heuristics must
work with doubles (the latter was also buggy for 32-bit clauses).

Now MAX_CLAUSE_LENGTH=64 incurs roughly a 10% speed penalty, and thus
default reset to 32.
Providing besides "tawSolver" (MAX_CLAUSE_LENGTH=32) also "tawSolver64".

Use "oklib --prebuild" to create links.
Improved the 3 names related to clause-lengths.
Removed macro MAX_CLAUSES (since actually not used).
MAX_VARS now a macro (which can be set for compilation).
Removed superfluous header.
Now only using "std::abs".