fix(cmake): generate version.h under PROJECT_BINARY_DIR to support add_subdirectory() builds

[Angelia-Wang]

What

CMakeLists.txt generates the versioned header at ${CMAKE_BINARY_DIR}/src/iceberg/version.h, but the include path used to compile the iceberg target (ICEBERG_INCLUDES in src/iceberg/CMakeLists.txt) is ${PROJECT_BINARY_DIR}/src. This change makes both consistently use PROJECT_BINARY_DIR.

Why

When iceberg-cpp is built as the top-level project, CMAKE_BINARY_DIR and PROJECT_BINARY_DIR happen to be the same directory, so this mismatch is invisible.

When iceberg-cpp is consumed via add_subdirectory() from another CMake project (e.g. vendoring it under third/iceberg-cpp/), CMAKE_BINARY_DIR resolves to the outermost project's build directory, while PROJECT_BINARY_DIR correctly resolves to the build directory nested under the outer project (e.g. <outer_build_dir>/third/iceberg-cpp/). The generated version.h then ends up in the wrong place and any translation unit that does #include "iceberg/version.h" (e.g. catalog/rest/constant.h, deletes/dv_util.cc, deletes/dv_writer.cc) fails with a missing header when building as a nested dependency.

How

Use PROJECT_BINARY_DIR instead of CMAKE_BINARY_DIR as the output path passed to iceberg_configure_version_header(), matching the directory already used by ICEBERG_INCLUDES.

Testing

• Standalone build: unaffected, since CMAKE_BINARY_DIR == PROJECT_BINARY_DIR at the top level.
• Verified by consuming this repo via add_subdirectory() from an external CMake project; the build previously failed to locate iceberg/version.h and now succeeds.

[wgtmac]

Good catch!