nut build fails on macos due to:
1. configure script can not use AR env var due to OpenWrt build
system provides only executable name (e.g. aarch64-openwrt-linux-musl-gcc-ar)
but configure script checks if AR has '/'. As a result, configure
script ignores AR env var and uses system `ar` but macos `ar` is
not compatible with the objects generated by OpenWrt GCC toolchain.

This commit explicitly sets ac_cv_path_AR=$(TARGET_AR) to use
OpenWrt toolchain AR.

2. configure script detects if build host is macos and adds
macosx_ups driver as a build target, but this driver can not be
build with OpenWrt toolchain because OpenWrt is Linux.

This commit explicitly disables macosx_ups driver using configure
flag --without-macosx_ups

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>