Issue 2406 dont build doc with low sphinx 3.3

[cvvergara]

Fixes #2406

@pgRouting/admins

[cvvergara]

Messages:

When version is too low

-- Found Sphinx: /home/vicky/pgrouting/pgrouting/env-pgr/bin/sphinx-build (Required is at least version "4.0") 
CMake Warning at doc/CMakeLists.txt:154 (message):
  Found Sphinx v1.0!

      This project requires Sphinx at least v4.0
      building without documentation

When sphinx not found

-- Could NOT find Sphinx (missing: SPHINX_EXECUTABLE SPHINX_VERSION_OK) (Required is at least version "4.0")
CMake Warning at doc/CMakeLists.txt:146 (message):
  Sphinx not found.

      building without documentation