Warn if checking programs are not installed

`make checkstyle` silently skips checks if the required programs are not
installed (e.g. shellcheck, mandoc).  Therefore developers may not
realize that they are not getting the full suite of code checks.  This
also applies to more specific targets like `make shellcheck`.

We should print a warning message when a check is skipped due to missing
tools.

Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed by: Pavel Zakharov <[email protected]>
Reviewed by: Prakash Surya <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #7984 

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