math/pdal: fix build on powerpc64

vec_sqrt() requires VSX:
/wrkdirs/usr/ports/math/pdal/work/PDAL-2.3.0/vendor/eigen/Eigen/src/Core/arch/AltiVec/PacketMath.h:1296:32: error: use of undeclared identifier 'vec_sqrt'; did you mean 'vec_rsqrte'?
  BF16_TO_F32_UNARY_OP_WRAPPER(vec_sqrt, a);
                               ^~~~~~~~
                               vec_rsqrte

math/pdal/Makefile

@@ -68,6 +68,8 @@ MBIO_CMAKE_BOOL=	BUILD_PLUGIN_MBIO
 PGPOINTCLOUD_CMAKE_BOOL=	BUILD_PLUGIN_PGPOINTCLOUD
 PGPOINTCLOUD_USES=	pgsql
 
+CXXFLAGS_powerpc64=	-mvsx
+
 post-install-BASH-on:
 	${MKDIR} ${STAGEDIR}${PREFIX}/share/bash-completion/completions
 	${INSTALL_DATA} ${WRKSRC}/scripts/bash-completion/pdal \