Enable link-time optimization in release builds

[foxbenjaminfox]

For release builds, it's probably worth enabling link-time optimization (LTO). The performance gains can be moderately significant: In the warm-cache-simple-pattern benchmark I observed an 8% improvement in the case of the [0-9]\.jpg$ pattern, and an 11% improvement in the case of the .*[0-9]\.jpg$ pattern. (Of course, much like the example benchmark in the README, this is one particular benchmark on one particular machine. I do wish there was a standardized set of files to run the benchmarks against, as I see was discussed (but not resolved) in issue #36.)

The main downside of LTO is that it makes the compile times longer. I feel that it's worth making that tradeoff in a tool like fd, for which (runtime) speed is one of its major selling points.

[sharkdp]

Thank you very much for your contribution. I didn't know LTO could be enabled this easily. This is definitely promising. I'd love to make a few benchmarks myself before merging this (might take a few days).

Regarding standardized benchmarks, we should definitely work on this. We have been using the Linux Kernel Source in a few tickets, for example.

[sharkdp]

I see consistent results (see below). Thank you so much!

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Simple pattern:

Command	Mean [ms]	Min…Max [ms]
fd-master -HI '.*[0-9]\.jpg$' '/home/shark'	605.0 ± 2.6	600.3…609.9
fd-lto -HI '.*[0-9]\.jpg$' '/home/shark'	567.7 ± 4.1	560.0…575.7
fd-master '.*[0-9]\.jpg$' '/home/shark'	196.4 ± 3.0	193.3…205.4
fd-lto '.*[0-9]\.jpg$' '/home/shark'	177.9 ± 1.3	175.9…179.8

Filtering by type:

Command	Mean [ms]	Min…Max [ms]
find '/home/shark' -type l	1869.4 ± 6.7	1862.7…1882.9
fd-master -HI '' '/home/shark' --type l	567.4 ± 2.8	563.4…572.1
fd-lto -HI '' '/home/shark' --type l	539.2 ± 2.6	535.0…543.4

Filtering by extension:

Command	Mean [ms]	Min…Max [ms]
find '/home/shark' -iname '*.jpg'	2344.4 ± 9.5	2331.3…2360.2
fd-master -HI --extension 'jpg' '' '/home/shark'	602.3 ± 1.9	599.5…604.4
fd-lto -HI --extension 'jpg' '' '/home/shark'	567.3 ± 4.0	563.1…574.8