fix: detect system Thrift in CONFIG mode

[abnobdoss]

What

REQUIRED_VARS dereferences its entries as variable names, so passing the target thrift::thrift resolves to nothing and the module reports Thrift as missing. Instead we need to pass a variable holding the target.

Why

The current behavior breaks -DICEBERG_BUNDLE_THRIFT=OFF against any Thrift shipping a CMake config, which is 0.13 and later. resolve_thrift_dependency() calls find_package(ThriftAlt MODULE REQUIRED), so the false negative aborts configure, and it is the path iceberg_hive recommends when the target is missing. It has gone unnoticed because Arrow's bundled Thrift defines the target first and the module returns early.

Validation

• Against a config-mode Thrift install, before: Could NOT find ThriftAlt (missing: thrift::thrift) and configure aborts. After: Found ThriftAlt: thrift::thrift and configure completes.

[wgtmac]

Good catch! Thanks for fixing this!