fix: force `lib` to be output dir for sdl on multilib linux systems

[remkade]

What does this PR address?

In some multilib linux systems cmake defaults to lib64 as the install lib path. This forces it to the expected lib. Has no effect on non-multilib systems since this just sets the default to the default.

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[remkade]

This was failing on my multilib gentoo linux install because cmake wanted to put the static libs into lib64 rather than lib as expected.

[aarnphm]

Thanks. I will find some time next week to release a new package for this.