feat(build.sh): Add a watch mode for automatic recompilation

[rhuss]

This features uses and requires https://github.com/AgentCosmic/xnotify
and should work on any platform

Use it with

• Watch & compile: build.sh --watch
• Watch, compile, test: build.sh --watch --test
• Watch, compile, test & verbose output: build.sh --watch --test --verbose

(shortcuts -w && -t can be used, too)

[knative-prow-robot]

Hi @rhuss. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[rhuss]

/assign cppforlife

[rhuss]

/ok-to-test

[knative-prow-robot]

@rhuss: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/ok-to-test

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[rhuss]

/ok-to-test

[rhuss]

@cppforlife @sixolet Could I please have a quick review and potential lgtm ? It's really a small change, for developer convenience. It's totally harmless as it doesn't have to be used and doesn't impact the compilation or release process. In a next step I would add a tempfs for watching which increase compile times ~40% as reported in https://daltontan.com/how-to-compile-go-code-faster-with-ram-disk/24

[sixolet]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rhuss, sixolet

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