These local copies of CheckForPthreads.c and FindThreads.cmake override
the ones included with CMake. These versions create CMake::Threads, but
Qt6 expects Threads::Threads created by CMake 3.1+. These local versions
seem to be based on old copies from CMake from late 2014 with some
customizations. Let's just use the built-in ones that CMake ships.
This commit also changes CMakeLists.txt files in UI and libobs to
require and link to Threads::Threads.
Co-authored-by: Kurt Kartaltepe <kkartaltepe@gmail.com>
ea9c75bacb