cmd/cgo: clarify implicit "cgo" build constraint

[ikkerens]

When using the special import "C", the "cgo" build constraint is implied for the go file, potentially triggering unclear "undefined" error messages. The documentation so far did not yet mention this.

Updates #24068, adds the instructed "quick win" as a more helpful message from the compiler is non-trivial.

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[ikkerens]

I signed it!

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Message from Ian Lance Taylor:

Patch Set 1:

(1 comment)

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Message from Rens Rikkerink:

Patch Set 2:

Patch Set 1:

(1 comment)

I've pushed the change as I agree with the more direct approach.

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Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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Uploaded patch set 4: Commit message was updated.

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Message from Ian Lance Taylor:

Patch Set 5: Commit message was updated.

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[ikkerens]

Closing as this has been merged.

[ianlancetaylor]

CC @andybons: how come this pull request had to be explicitly closed, and was not automatically closed when I submitted the issue in Gerrit?

[andybons]

Hm. It should have been closed by GerritBot.