Differentiate between lib and lib64 in the build settings.

[mkoncek]

See: https://docs.fedoraproject.org/en-US/packaging-guidelines/RPMMacros/#macros_installation
In Fedora, the default library path for the x86_64 architecture is /usr/lib64. I would like to be able to export some variable to before the build to allow rake install to use that path.

[dearblue]

If simply replacing lib/ directory with lib64/ during rake install, lib/libmruby.mak and bin/mruby-config will not work.
We need to replace the embedded lib/ with a variable so that it can be determined at runtime.
And we may also need a mechanism to replace the content of files when they are installed (copied).

If no one else is working on it, I may eventually PR it.

[rubyFeedback]

I understand the use case. Personally I still find it wrong to label lib-directories with versions. /usr/lib/ always seemed to make more sense than /usr/lib32/ and /usr/lib64/, but as said, I understand the use case and also agree that build systems should support this. Another awkward thing is versioned binaries, such as /usr/bin/python3.11 or /usr/bin/python3.12. I also understand the use cases here as well (/usr/bin/python is then typically a symlink) but I feel this is all not really an elegant solution.