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