Fix a warning interpreted as error when building the Linux kernel
on macOS host, by setting MACOSX_DEPLOYMENT_TARGET to the version
of the host. Fixes:
make[5]: Entering directory '/Volumes/test/openwrt/build_dir/target-x86_64_musl/linux-x86_64/linux-6.18.38'
HOSTCC scripts/mod/modpost.o
scripts/mod/modpost.c:1719:9: error: 'strchrnul' is only available on macOS 15.4 or newer [-Werror,-Wunguarded-availability-new]
1719 | sep = strchrnul(namespace, ',');
| ^~~~~~~~~
Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24251
Signed-off-by: Robert Marko <robimarko@gmail.com>
