libnl: Fix building with uClibc

libnl: Fix building with uClibc

uClibc doesn't implement strerror_l() and thus libnl starting from
3.2.29 couldn't be compiled with it any longer, see
https://github.com/thom311/libnl/commit/6c2d111177e91184073c44f83d4a6182aaba06d7

To work-around that problem we'll just do a check on strerror_l()
availability during configuration and if it's not there just fall back
to locale-less strerror().

Patch for libnl is alreadfy merged upstream, see
https://github.com/thom311/libnl/commit/e15966ac7f3b43df2acf869f98089762807d0568
and once the next libnl release happens this one must be removed from
Lede/OpenWrt.

Signed-off-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: John Crispin <john@phrozen.org>
Cc: Daniel Engberg <daniel.engberg.lists@pyret.net>