package/libbsd: enable for non-glibc toolchains

package/libbsd: enable for non-glibc toolchains

libbsd builds now almost fine with a musl or uClibc toolchain, except
for one issue introduced in the latest version bump. Upstream commit
22fbd62368c39de8ac5e249d1502d5ac0ffdef30 [1] uses the glibc-only macro
`__GLIBC_PREREQ`. The issue is fixed by the attached patch from upstream,
which fixes the use of `__GLIBC_PREREQ` on non-glibc toolchains.

Backported from:
https://cgit.freedesktop.org/libbsd/patch/?id=1f8a3f7bccfc84b195218ad0086ebd57049c3490

netcat-openbsd is the only package selecting libbsd. However, building
it still needs a glibc toolchain, as it uses `b64_ntop` which is not
available in musl or uClibc.

Build has been successfully tested with:
 * armv7-eabihf--glibc--bleeding-edge-2017.11-1
 * armv7-eabihf--musl--bleeding-edge-2018.02-1
 * armv7-eabihf--uclibc--bleeding-edge-2018.02-1

[1] https://cgit.freedesktop.org/libbsd/commit/?id=22fbd62368c39de8ac5e249d1502d5ac0ffdef30

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>