Compilation warning during compilation on Mac OSX

[LeventErkok]

In case this is of interest:

In file included from ../src/math/lp/numeric_pair.h:26:
../src/util/rational.h:135:16: warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
    rational & operator=(rational&&) = default;
               ^
../src/util/rational.h:24:11: note: move assignment operator of 'rational' is implicitly deleted because field 'm_val' has a deleted move
      assignment operator
    mpq   m_val;
          ^
../src/util/mpq.h:33:11: note: 'operator=' has been explicitly marked deleted here
    mpq & operator=(mpq const &) = delete;
          ^
1 warning generated.

[LeventErkok]

Note: The commit you put in does not get rid of the warning message.