No googletest auto-download

[ursfassler]

Summary

Use find-gtest from cmake and don't automatically download gtest.

Details

Use same method for gtest as for the other external dependencies; they have to be provided externally.
Increase needed versions of cmake to 3.9 and gtest to 1.11.0.

Motivation and Context

This change is motivated by:

• Use find-gtest from cmake, no special solution
• use same strategy for all external dependencies (no automatic download)
• automatic download of gtest was troublesome in some projects where gtest is already used

How Has This Been Tested?

• automated tests
• run against project using cucumber-cpp

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

It may break some projects because googletest has to be provided. Code is not affected.

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to main, keeping only relevant commits.