package/exim: remove libnsl linking

package/exim: remove libnsl linking

glibc now considers its built-in libnsl as being obsolete, and requires
passing --enable-obsolete-libnsl to have it built and installed. libnsl
is now provided as a separate project [1], but it isn't packaged yet in
Buildroot.

In preparation for dropping --enable-obsolete-libnsl from the glibc
package, this commit ensures that exim doesn't use libnsl. It was
already the case for uclibc and musl toolchains, so this commit simply
extends that to make sure libnsl is also not used with glibc toolchains.

Only Exim's nis.so and nisplus.so lookup modules require libnsl,
but they are not build by default. So we can safely remove -lnsl
from the Makefile-Linux. If someone want these modules, a new libnsl
package must be added first to provide nsl library.

Since we remove -lnsl unconditionally, use a patch instead of a sed command.

[1] https://github.com/thkukuk/libnsl.git
[2] https://fedoraproject.org/wiki/Changes/NISIPv6

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>