Revision 433825

lang/yap{,-devel}: use clang and unbreak with gcc5 or later

In file included from ./Solver.C:20:
In file included from ./Solver.h:29:
./SolverTypes.h:122:20: error: friend declaration specifying a default argument must be a definition
    friend Clause* Clause_new(const V& ps, bool learnt = false);
                   ^
./SolverTypes.h:147:9: error: friend declaration specifying a default argument must be the only
      declaration
Clause* Clause_new(const V& ps, bool learnt) {
        ^
./SolverTypes.h:122:20: note: previous declaration is here
    friend Clause* Clause_new(const V& ps, bool learnt = false);
                   ^
./Solver.C:123:21: error: no matching function for call to 'Clause_new'
        Clause* c = Clause_new(ps, false);
                    ^~~~~~~~~~
./Solver.C:602:29: error: no matching function for call to 'Clause_new'
                Clause* c = Clause_new(learnt_clause, true);
                            ^~~~~~~~~~
4 errors generated.

PR:		216809
Obtained from:	upstream
Approved by:	portmgr blanket