php80 & php81: correct zts option invocation

Starting with PHP 8.0, zts is enabled with --enable-zts, rather than
the older --enable-maintainer-zts. Addresses PR pkg/56681.

The actual pkgsrc option name should probably be adjusted, and the
current "maintainer-zts" moved to PKG_LEGACY_OPTIONS. I've left it for
another commit.

(gutteridge)