Makefile: need better bootstrap regen detection

[maddyblue]

The recent addition of protoc-gen-doc (#51803) caused people to have problems building:

./bin/protoc-gen-doc: program not found or is not executable

Running rm bin/.bootstrap fixes this. We need some better detection to regenerate .bootstrap when there are new programs added.

Epic CRDB-10447

Jira issue: CRDB-3944

[knz]

The problem here is twofold:

• the various utilities in bin/ do not depend on the .bootstrap target
• the various uses of the utilities in bin/ do not depend on the executable in bin/ and thus not on .bootstrap.

So if we introduce a new utility and just change the action for .bootstrap, "nothing happen" and the uses of the new utility fail until the user explicitly refreshes .bootstrap.

IMHO it would be time-consuming and error-prone to establish all these dependencies at a fine grain. I have a better suggestion:

• extract the rule that builds .bootstrap into a separate makefile bootstrap.mk
• make the .bootstrap depend on boostrap.mk
• make the various uses of utilities depend on .bootstrap

This way if we ever modify bootstrap.mk to add a new go install line, that will force a refresh of .bootstrap upon first use of a utility.

[jordanlewis]

No longer relevant given bazel.