[Bug 261807] Mk/Uses/cmake.mk: Add cmake:lto option

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261807

--- Comment #7 from Jan Beich <jbeich at FreeBSD.org> ---
(In reply to Yuri Victorovich from comment #3)
> Port option LTO can then be defined with LTO_USES=cmake:lto.

Appending arguments to an existing USES=foo is broken, see bug 199603.

(In reply to Max Brazhnikov from comment #6)
> LTO should never be an option!

See https://cgit.freebsd.org/ports/log/?qt=grep&q=LTO+by+default
LTO is similar to OPTIMIZED_CFLAGS option but slightly more fragile and
increases build time, so can get in the way with unusual toolchain
configuration or when bisecting. Blindly enabling -flto can cause crashes at
runtime e.g., https://bugs.llvm.org/show_bug.cgi?id=40635

Modern PLs like Rust enable LTO for user code as part of default optimizations,
see https://doc.rust-lang.org/rustc/codegen-options/index.html#lto

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