Feature: Warn on missing task definitions

[mcandre]

checkmake has some odd behavior regarding phony targets. It specifies several minimum phony targets, such as all, clean, and test. But it doesn't actually validate whether the tasks are defined.

(GNU) make doesn't appear to care about undefined tasks until very late, when the task is explicitly invoked.

A linter such as checkmake would be a natural place to insert a new rule, that validates that all tasks referenced in a makefile, are actually defined somewhere.