gapbind14: unconditionally call __builtin_unreachable

[james-d-mitchell]

This change resolves the compiler warnings from the previous version of the code:

gapbind14/include/gapbind14/cpp_fn.hpp:42:3: warning: function declared 'noreturn' should not return [-Winvalid-noreturn]
   42 |   }

when compiling with clang. It also stops gapbind14 from depending on the Semigroups package at all (by removing the include of the config.hpp file).

We should probably update the build system to error out if __builtin_unreachable isn't present.