Support building using "computed gotos" for clang-cl on Windows #131278

Per suggestion from @zooba in #130040 (comment), don't be too fancy here:

• just support a new optimizing build flag WITH_COMPUTED_GOTOS and keep build.bat out of the loop. It does not have to learn such flags. It already knows too many of them "which could just be passed through".
• only test for != '' rather than a specific value.
• no magic defaults - explicit is better than implicit. If it is there, it's enabled, else it's disabled.
• but document like set WITH_COMPUTED_GOTOS=true
• this would give us the possibility to opt out using WITH_COMPUTED_GOTOS=false, should we ever enable it by default (and the user does not like our guess)

Linked PRs

• GH-131278: Support building using "computed gotos" for clang-cl on Windows #131279