[birolcapa]

Avoids unnecessary OpenBLAS rebuilds during reconfigure

[martin-frbg]

Hmm, it's not quite clear to me when this change would actually be useful ?

[birolcapa]

Hmm, it's not quite clear to me when this change would actually be useful ?

Thank you for your review. I appreciate it.

This is useful when CMake reconfigures because of an unrelated CMake change. The file(RENAME... call replaces config.h every time, even its contents are unchanged, so its timestamp changes. Since OpenBLAS sources depend on config.h, the build system sees the header as newer and recompiles OpenBLAS unnecessarily.

file(COPY_FILE ... ONLY_IF_DIFFERENT) preserves the existing config.h timestamp when the generated contents are identical.

This matters especially in incremental builds, where a small CMake change elsewhere will not trigger building OpenBLAS.

But now I recognized a problem with my PR, OpenBLAS asks cmake_minimum_required(VERSION 3.16.0), but file(COPY_FILE ... ONLY_IF_DIFFERENT) syntax is added in version 3.21.

Am I allowed to change required CMake version? If you agree, I can do that.