make: fix validate-docs

[dmke]

In #1374 the PDNS documentation was amended, but the author forgot to re-generate the docs.

We do have a validation check in the main workflow, but it didn't catch the change because the underlying Makefile target was broken.

This commit replaces the Makefile conditional with an equivalent shell conditional.

Explanation

The conditional checking for a dirty working directory ran before generating the documentation. In effect, make validate-doc only
did the right thing on the second invokation.

Consider this simplified Makefile example:

generate:
	aaaa
	
validate: generate
ifneq ($(shell bbbb),)
	cccc
else
	dddd
endif

The order of evaluation for make validate is:

1. detect generate target, no dependencies,
2. set body for generate target to aaaa,
3. detect validate target, with dependency generate,
4. execute bbbb, compare with empty string,
   • if not empty: set body for validate target to cccc
   • else: set body for validate target to dddd
5. invoke generate → execute aaaa (generate is a dependency of validate),
6. invoke validate → execute cccc or dddd.

This also means that step 4 is also (unnecessarily) executed when the validate target is never invoked.

[ldez]

LGTM