LOCAL: use GNU-specific strerror_r() only if __GLIBC__ is not defined

This makes musl happy.
musl always chooses POSIX strerror_r().

str_error(3) suggests to use this condition:

(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE

but it doesn't work with musl. The only way to check is if __GLIBC__
is defined.

Change-Id: I30b508be65bfabd78cdbd73f63bcb222435cbcd5