make check fails when you do not have docs building/testing requirements (build works)

[strk]

In stable-3.4 branch you can ./configure just fine and even "make" but "make check" fails complaining about a missing "xmllint".

I'd prefer for "make check" to just skip testing documentation when documentation cannot be built instead of erroring out.

[strk]

Same for xsltproc:

---- Making check-unit in doc
make[1]: Entering directory '/usr/local/src/postgis/postgis-3.4/build/pg14/doc'

configure was unable to find 'xsltproc' which is required
to build the documentation.
Install xsltproc and then re-run configure. Alternatively
refer to online manual:

  http://postgis.net/documentation

Build (make) works:

---- Making all in doc
make[1]: Entering directory '/usr/local/src/postgis/postgis-3.4/build/pg14/doc'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/usr/local/src/postgis/postgis-3.4/build/pg14/doc'
PostGIS was built successfully. Ready to install.