Use system googletest dependency when available

[topazus]

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[gchatelet]

@Mizux does that look good to you?

[Mizux]

my 2 cents,

1. GTest::gtest seems to be available only since CMake 3.5 (cpu_feature still use 3.0 as minimum requirement...)
   ref: https://cmake.org/cmake/help/v3.5/module/FindGTest.html
   ref: https://cmake.org/cmake/help/v3.4/module/FindGTest.html

2. Googletest didn't provide these aliases until recently
   issue: Add CMake aliases googletest#2429
   PR: CMake: Add namespaced ALIAS library googletest#3155
   commit: google/googletest@6c5c455
   available in v1.14.0 … release-1.11.0

to compare with https://repology.org/project/gtest/badges (e.g. ubuntu 20.04 LTS => gtest 1.10.0)

3. If your superbuild is using add_subdirectory() OR fetch_content() (without the new flag FIND_PACKAGE_ARGS NAMES ...) then you find_package() won't work as expected (i.e. ignore the call since your super already provide it)
   ref: https://cmake.org/cmake/help/latest/module/FetchContent.html#integrating-with-find-package

4. Also in 3.24 we would be able to have FetchContent calling find_package() before sourcing if unavailable...
   ref: https://cmake.org/cmake/help/latest/module/FetchContent.html#variable:FETCHCONTENT_TRY_FIND_PACKAGE_MODE

[Mizux]

EDIT:

1. DONE (we now target cmake 3.13)
2. googletest v1.11 (June 2021) should provide alias -> Can we suppose super build use at least googletest 1.11 ?

[Mizux]

since you add REQUIRED -> if not found the cmake configure will fail where instead you would like to fallback to "build googletest if not found in the system"