fix build error on ARM due to wrong glibc check

the test for glibc < 2 "succeeds" wrongly for any non-glibc C library,
and breaks the build on musl libc.
we must first test if __GLIBC__ is defined at all, before using it
unconditionally.

Signed-off-by: John Spencer <maillist-qemu@barfooze.de>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
