ext/zip: Replace $LIBZIP_LIBDIR with $LIBZIP_LIBS

[hughmcmaster]

No description provided.

[nikic]

Technically this seems "less correct", because we're going to duplicate -lzip both from the extra libs argument and the actually tested library. But I guess we don't really care anyway and this is shorter...

[hughmcmaster]

@nikic, That duplication would happen even if $ZIP_SHARED_LIBADD was passed in. The actual problem is that $LDFLAGS (and $LIBS) is empty when building shared extensions. If you manually set LDFLAGS, everything works as expected. But I'm struggling to work out why those variables are empty.

[nikic]

@hughmcmaster I believe that's because those LDFLAGS are global, while here you want to add to add extra libraries only for one extension, not all of them or the main PHP build.