LD_PRELOAD sometimes forces loading libraries into unrelated
executables. For example, macOS on recent Apple Silicon can execute
arm64 and arm64e binaries by default, and /usr/bin/clang is built as
arm64e. If Ruby is built as arm64, and mkmf launched through runruby.rb
spawns /usr/bin/clang, dynamic loader tries to load libruby (arm64e)
into clang (arm64). This force-load causes library load failure.
In theory, we don't need both LD_PRELOAD and LD_LIBRARY_PATH at the same
time, because executables requiring libruby already have libruby
dependency, so LD_LIBRARY_PATH is enough for this case.