laurent.cc: don't use const vectors as map keys
Apparently, Sun's compiler has a problem with those:
"/solapplic/sun-studio-11/SUNWspro/prod/include/CC/Cstd/./map", line 251: Error: Multiple declaration for std::map<const std::vector<int>, const _new__evalue*, std::less<const std::vector<int>>, std::allocator<std::pair<const std::vector<int>, const _new__evalue*>>>::insert(const std::pair<const std::vector<int>, const _new__evalue*>&).
"../../git/barvinok/laurent.cc", line 225: Where: While specializing "std::map<const std::vector<int>, const _new__evalue*, std::less<const std::vector<int>>, std::allocator<std::pair<const std::vector<int>, const _new__evalue*>>>".
"../../git/barvinok/laurent.cc", line 225: Where: Specialized in non-template code.