clang on macOS and overzealous warnings

[Greg Troxel <gdt%lexort.com@localhost>]

While building sysutils/bup on macOS 15, bup's configure (which is not
autoconf) tries to build a small program that invokes a libc routine.
That's pretty normal; it works on other platforms and it works when
building bup on macOS not under pkgsrc.


Under pkgsrc, it errors out:

  checking for the mincore function
  clang: error: argument unused during compilation: '-L/opt/n0/pkgsrc/sysutils/bup/work/.buildlink/lib' [-Werror,-Wunused-command-line-argument]
  clang: error: argument unused during compilation: '-L/opt/n0/pkgsrc/sysutils/bup/work/.buildlink/lib' [-Werror,-Wunused-command-line-argument]
  clang: error: argument unused during compilation: '-L/opt/n0/pkgsrc/sysutils/bup/work/.buildlink/lib' [-Werror,-Wunused-command-line-argument]

The problem is that pkgsrc adds -L for the bl3 lib dir, always.  In my
view that's fine.  But clang objects, I think because there was no -lfoo
later that could have used the search path.

(Separately, I find I need USE_CWRAPPERS=no, or to alter the definition
of cpp.)

I wonder:

  - does sysutils/bup build for anyone else?
  - does this break other packages?
  - opinions about what to do?