Reverting the build mode to 'builtin' to be able to link EPOS.

Strangely, one extra edit was necessary: changing from
    extern "C" { void * _end; }
to
    extern "C" void * _end;
in file src/init/init_application.cc. I tought these were equivalent;
apparently, the first form is a definition while the second form
is merely a declaration.