Log Message:
Re-add editors/xemacs/hacks.mk --

gcc 5 builtins collide with src/gmalloc.c's calloc(), which
results in 'xemacs -vanilla' busy-looping during the build.

Joerg suggested building with -fno-builtin, see
<http://mail-index.netbsd.org/tech-toolchain/2016/10/28/msg002820.html> ff.