security/botan2: fix build on powerpc64

In file included from src/lib/block/aes/aes_power8/aes_power8.cpp:15:
/usr/lib/clang/18/include/altivec.h:17365:10: error: '__builtin_altivec_crypto_vcipher' needs target feature power8-vector
 17365 |   return __builtin_altivec_crypto_vcipher(__a, __b);
       |          ^

security/botan2/Makefile

@@ -78,6 +78,9 @@ SQLITE3_CONFIGURE_WITH=	sqlite3
 CONFIGURE_ARGS+=	--cc-abi="-march=armv8-a+crypto"
 .elif ${ARCH} == powerpc64
 CONFIGURE_ARGS+=	--cpu="ppc64"
+.if !defined(MACHINE_CPU) || (defined(MACHINE_CPU) && !${MACHINE_ABI:Mvsx2})
+CONFIGURE_ARGS+=	--disable-powercrypto
+.endif
 .elif ${ARCH} == powerpc64le
 CONFIGURE_ARGS+=	--cpu="ppc64le"
 .endif