Before getting into the details here about why we need to read `libA.so` at build time: Perhaps the documentation gentoo provides about `ROOT` that was noted by rini17 is incorrect/misleading and `ROOT` is used as the source of build time dependencies automatically and the documentation was intended for ebuild writers and not emerge users? (in which case, Gentoo should fix/clarify it).

"link" has 2 meanings:

1. As you've pointed at, at runtime we have linking occur that reads files.

2. Additionally, building software with `gcc`/etc, we tell it what libraries it needs to link against (`-lA`). When we do that, `gcc` as part of the build process searches for `libA.so` (in various locations dependent on its configuration and arguments), reads the `libA.so` file, and uses that info to determine what to emit in the object file that `gcc` is building. So we need to read a shared library. The same is true for headers used at build time. How much this actually ends up mattering varies depending on architectural details, and whether libraries have different symbols exposed (or generate different header files) depending on what the bit-width of time_t is. It may well be that for most packages we could have the build-time linking occur against a 32-bit time_t object as long as runtime linking is done to the 64-bit time_t object (assuming one is using identical package versions or at least versions with a compatible ABI).