package/zlib-ng: fix build on powerpc

Handle WITH_POWER8 to fix the following build failure with powerpc and
libglib2 raised since bump to version 1.9.9-b1 in commit
1f7b12a0b4572d7e763c431a63c22cfb912b8b14 and
https://github.com/zlib-ng/zlib-ng/commit/b81f4ee96dcbdf1db34b00727b6f1829a2ba1edb:

/home/autobuild/autobuild/instance-9/output-1/host/lib/gcc/powerpc-buildroot-linux-musl/11.2.0/../../../../powerpc-buildroot-linux-musl/bin/ld: /home/autobuild/autobuild/instance-9/output-1/host/powerpc-buildroot-linux-musl/sysroot/usr/lib/libz.a(adler32_power8.c.o): in function `adler32_power8':
/home/autobuild/autobuild/instance-9/output-1/build/zlib-ng-2.0.6/arch/power/adler32_power8.c:106: undefined reference to `vec_sumsu'

Fixes:
 - http://autobuild.buildroot.org/results/f864a0e2307e25342b45c90a5b4fb57a0f5c1c93

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 192dfc68c0e43d0865beb16250582b3b4e5ed178)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/zlib-ng/zlib-ng.mk

@@ -23,4 +23,10 @@ ifeq ($(BR2_arm),y)
 ZLIB_NG_CONF_OPTS += -DWITH_ACLE=1 -DWITH_NEON=1
 endif
 
+ifeq ($(BR2_powerpc_power8),y)
+ZLIB_NG_CONF_OPTS += -DWITH_POWER8=ON
+else
+ZLIB_NG_CONF_OPTS += -DWITH_POWER8=OFF
+endif
+
 $(eval $(cmake-package))