build: use -pthread when possible

Using -pthread is the "right way" of enabling threading support in GCC
and similar, and this can accomplished with the
THREADS_PREFER_PTHREAD_FLAG option. Its use is also recommended by the
CMake devs, see
https://cmake.org/cmake/help/latest/module/FindThreads.html#variable:THREADS_PREFER_PTHREAD_FLAG

CMakeLists.txt

@@ -183,6 +183,7 @@ zyan_set_source_group("Zycore")
 
 if (${CMAKE_SYSTEM_NAME} STREQUAL "Linux" AND NOT ZYAN_NO_LIBC)
     target_compile_definitions("Zycore" PRIVATE "_GNU_SOURCE")
+    set(THREADS_PREFER_PTHREAD_FLAG ON)
     find_package(Threads REQUIRED)
     target_link_libraries("Zycore" Threads::Threads)
 endif ()