`-Ofast` has an effect on the defaults for `-ffast-math` (documented
before this patch), and `-fstrict-aliasing` (not documented before this
patch).
On some platforms, `-Ofast` cannot be replaced with `-O3 -ffast-math`,
because the strict aliasing default would change. `-Ofast` can only be
replaced (in the exact same position) with `-O3 -ffast-math
-fstrict-aliasing` if `-Ofast` is the effective optimization level
(i.e., it is not followed by another `-O<value>` flag). Otherwise, the
`-Ofast` flag should just be deleted as it is having no effect.
This is all too difficult to summarise in a warning message, so this PR
mostly updates the docs. We keep the message about "use `-O3` to get
conforming optimizations" in the hope this encourages people to adopt
`-O3` alone.
The warning message is now emitted any time there is `-Ofast` in the
command-line string, rather than only when `-Ofast` is the effective
optimization level.
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