Openwrt now supports only glibc and musl. Add support for musl and
rework the libc check to handle the new config flags and correctly
compile package basend on that.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 7be01fe13b4517e5edb8a4818f437d60144cdcb4)
