More fixes for builddir != srcdir.

More fixes for builddir != srcdir.

 - Use absolute directories for $builddir and $srcdir in the Makefile

 - Don't try and combine source files in $builddir and $srcdir to build
   proto.h.  It's just too hard to get it right across all targets we
   wish to compile on.  Use a hand created prototype for the single
   function in smbd/build_options.c that we need.  This allows us to ditch
   all the extra sed work that was causing problems: \t not portable - hah!

 - Fix bogus delheaders target to remove the correct files

This appears to work quite nicely now.  Let's see how it goes on the
buildfarm machines.