Fix CI: Link to Pthread library explicitly

tests/signal/CMakeLists.txt

@@ -13,6 +13,12 @@ project(test-signal VERSION 0.1 LANGUAGES CXX)
 add_executable(${PROJECT_NAME}
     tst_signal.cpp
 )
-target_link_libraries(${PROJECT_NAME} KDAB::KDBindings)
+# For some reason, CMake with gcc doesn't automatically include the pthread library
+# when using std::thread. This is a workaround for that.
+# IMHO, this is ridiculous, std::thread is part of the standard library, it should just work
+# when I use C++, but it is what it is.
+# See: https://cmake.cmake.narkive.com/wWDhK9RQ/undefined-reference-to-pthread-create
+find_package (Threads)
+target_link_libraries(${PROJECT_NAME} KDAB::KDBindings ${CMAKE_THREAD_LIBS_INIT})
 
 add_test(${PROJECT_NAME} ${PROJECT_NAME})