build.rs: search /lib64 as well

[hinto-janai]

Description

Search /lib64 in build.rs in addition to /lib.

Motivation and Context

Building on Fedora needs /lib64 specified instead of /lib, else a linking error like this occurs: https://github.com/Cuprate/cuprate/actions/runs/15330922254/job/43137221458#step:9:26.

How Has This Been Tested?

cargo build on Fedora 42 and Debian 12 before/after change.

[lnchan]

Hi, just to add extra context: the reason this happens is because of CMake's implementation of the GNU Coding Standard's Installation Directories, which is described in the GNUInstallDirs module.

Debian-based distributions (Debian, Ubuntu, Linux Mint and others) are the only cases where lib/ would be the target LIBDIR when natively compiling on a 64-bit system, meaning no other Linux distribution would be able to build randomx-rs properly.

Copy-pasting the CMake logic for the default LIBDIR definition:

  # Override this default 'lib' with 'lib64' iff:
  #  - we are on Linux system but NOT cross-compiling
  #  - we are NOT on debian
  #  - we are on a 64 bits system
  # reason is: amd64 ABI: http://www.x86-64.org/documentation/abi.pdf
  # For Debian with multiarch, use 'lib/${CMAKE_LIBRARY_ARCHITECTURE}' if
  # CMAKE_LIBRARY_ARCHITECTURE is set (which contains e.g. "i386-linux-gnu"
  # and CMAKE_INSTALL_PREFIX is "/usr"

While this does fix things on Fedora for me as well (and this fix or a similar fix is required for Tari projects to build on my Fedora setup), I feel this will still break: if cross-compiling, the LIBDIR would not be lib64/ but triples like x86_84-linux-gnu, causing the same problem again.

Sadly, that seems to be an issue with cmake-rs itself, and seems to be only possible to solve if the default CMakeLists does not allow overriding the install(...)'s library destination, meaning adding this hardcoded search path is the "official solution".

[hansieodendaal]

I checked the Windows build locally, which did not give any problems, however, we have some clippy issues: cargo lints clippy --all-targets --all-features.