tests: allow using system gtest copy

[thesamesam]

Distributions often do builds with no network access available for both security reasons and also to ensure reproducibility.

This change tells CMake to query the system for a copy of gtest, but if it's not available, it'll fall back to downloading via FetchContent.

[serge1]

Accepted. Thank you!

[thesamesam]

Thanks!

[serge1]

Unfortunately, the change does not go plain. Here is my build log:

user@linux-on-jaguar:/.local/share/Trash/files$ cd /ELFIO/
user@linux-on-jaguar:/ELFIO$ mkdir build && cd build
user@linux-on-jaguar:/ELFIO/build$ cmake .. -DELFIO_BUILD_TESTS=ON
-- The C compiler identification is GNU 11.3.0
-- The CXX compiler identification is GNU 11.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at /usr/share/cmake-3.22/Modules/ExternalProject.cmake:2806 (message):
At least one entry of URL is a path (invalid in a list)
Call Stack (most recent call first):
/usr/share/cmake-3.22/Modules/ExternalProject.cmake:3716 (_ep_add_download_command)
CMakeLists.txt:15 (ExternalProject_Add)

-- Configuring incomplete, errors occurred!
See also "/home/user/ELFIO/build/_deps/googletest-subbuild/CMakeFiles/CMakeOutput.log".

CMake Error at /usr/share/cmake-3.22/Modules/FetchContent.cmake:1075 (message):
CMake step for googletest failed: 1
Call Stack (most recent call first):
/usr/share/cmake-3.22/Modules/FetchContent.cmake:1216:EVAL:2 (__FetchContent_directPopulate)
/usr/share/cmake-3.22/Modules/FetchContent.cmake:1216 (cmake_language)
/usr/share/cmake-3.22/Modules/FetchContent.cmake:1259 (FetchContent_Populate)
tests/CMakeLists.txt:12 (FetchContent_MakeAvailable)

-- Configuring incomplete, errors occurred!
See also "/home/user/ELFIO/build/CMakeFiles/CMakeOutput.log".

[serge1]

I have reverted the change for the meantime. Please resubmit after the fix

[thesamesam]

Thanks, I'll give it another look, it definitely fetched for me.

[serge1]

While testing, please don't forget to turn on the testing: ELFIO_BUILD_TESTS=ON

[thesamesam]

Oh dear. It's because it needs CMake 3.24 minimum.

[serge1]

The library switched to a newer 'googletest' archive with your change in