undefined reference to `testing::UnitTest::Run() - c++

Question

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add_executable(test_labyrinthe test_labyrinthe.cpp)
add_test(test_labyrinthe.cpp test_labyrinthe)
target_link_libraries(test_labyrinthe ${GTEST_LIBRARIES})
This is what I have in Test/CMakeLists.txt
cmake_minimum_required(VERSION 3.5)
project(TP1)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(SOURCE_FILES
Labyrinthe.cpp
Labyrinthe.h
Piece.cpp
Piece.h
Porte.cpp
Porte.h
Principal.cpp)
add_executable(TP1 ${SOURCE_FILES})
###########
## GTEST ##
###########
option(BUILD_TESTING "build unit tests" ON)
if(BUILD_TESTING)
include(ExternalProject)
ExternalProject_add(gtest-target
GIT_REPOSITORY "https://github.com/google/googletest"
CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=${CMAKE_CURRENT_BINARY_DIR}/extern"
UPDATE_COMMAND ""
)
include_directories(${CMAKE_CURRENT_BINARY_DIR}/extern/include)
link_directories(${CMAKE_CURRENT_BINARY_DIR}/extern/lib)
set(GTEST_LIBRARIES pthread gtest gmock gtest_main gmock_main)
enable_testing()
add_subdirectory(test)
endif()
This is what I have in the main CMakeLists.txt. Keep in mind that I have the only line #include <gtest/gtest.h> in test_labyrinthe.cpp.
When I run cmake .. and make, I got
[ 53%] Built target gtest-target
[ 86%] Built target TP1
Scanning dependencies of target test_labyrinthe
[ 93%] Building CXX object test/CMakeFiles/test_labyrinthe.dir/test_labyrinthe.cpp.o
[100%] Linking CXX executable test_labyrinthe
/usr/bin/ld: /home/infinity/ULaval/Session_Hiver_2021/Algorithmes_et_Structures_de_donnees/Travaux_Pratiques/TP1/CodeTP1/extern/lib/libgtest_main.a(gtest_main.cc.o): in function `main':
gtest_main.cc:(.text+0x3a): undefined reference to `testing::InitGoogleTest(int*, char**)'
/usr/bin/ld: /home/infinity/ULaval/Session_Hiver_2021/Algorithmes_et_Structures_de_donnees/Travaux_Pratiques/TP1/CodeTP1/extern/lib/libgtest_main.a(gtest_main.cc.o): in function `RUN_ALL_TESTS()':
gtest_main.cc:(.text._Z13RUN_ALL_TESTSv[_Z13RUN_ALL_TESTSv]+0x9): undefined reference to `testing::UnitTest::GetInstance()'
/usr/bin/ld: gtest_main.cc:(.text._Z13RUN_ALL_TESTSv[_Z13RUN_ALL_TESTSv]+0x11): undefined reference to `testing::UnitTest::Run()'
collect2: error: ld returned 1 exit status
make[2]: *** [test/CMakeFiles/test_labyrinthe.dir/build.make:84: test/test_labyrinthe] Error 1
make[1]: *** [CMakeFiles/Makefile2:152: test/CMakeFiles/test_labyrinthe.dir/all] Error 2
make: *** [Makefile:95: all] Error 2
This is really weird. Not sure how to make this work. Can you help?
EDIT When I run make test, I got
└╼ make test
Running tests...
Test project /home/infinity/ULaval/Session_Hiver_2021/Algorithmes_et_Structures_de_donnees/Travaux_Pratiques/TP1/CodeTP1
Start 1: test_labyrinthe.cpp
Could not find executable test_labyrinthe
Looked in the following places:
test_labyrinthe
test_labyrinthe
Release/test_labyrinthe
Release/test_labyrinthe
Debug/test_labyrinthe
Debug/test_labyrinthe
MinSizeRel/test_labyrinthe
MinSizeRel/test_labyrinthe
RelWithDebInfo/test_labyrinthe
RelWithDebInfo/test_labyrinthe
Deployment/test_labyrinthe
Deployment/test_labyrinthe
Development/test_labyrinthe
Development/test_labyrinthe
Unable to find executable: test_labyrinthe
1/1 Test #1: test_labyrinthe.cpp ..............***Not Run 0.00 sec
0% tests passed, 1 tests failed out of 1
Total Test time (real) = 0.00 sec
The following tests FAILED:
1 - test_labyrinthe.cpp (Not Run)
Errors while running CTest
make: *** [Makefile:84: test] Error 8

Answer 1

I believe the underlying problem is:
Keep in mind that I have the only line #include <gtest/gtest.h> in test_labyrinthe.cpp.
You'll want to add an actual test case to the file.
test_labyrinthe.cpp:
#include <gtest/gtest.h>
TEST(SomeTest, ABasicTest) {
EXPECT_EQ(1, 1);
}
Once you do that you'll probably run into some pthread linker errors. Changing to link pthread after gtest should resolve that.
set(GTEST_LIBRARIES gtest gmock gtest_main gmock_main pthread)

Answer 2

After uncountable tries, the solution was to add -pthread as you see here:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -pthread")