cmake: try to detect threads library

While we already make use of the variable `${CMAKE_THREAD_LIBS_INIT}`,
it is actually undefined due to us never including the "FindThreads"
module in the CMakeLists.txt. It is rather curious as to why this has
never triggered any error up to now, but it does in fact result in
linking errors on some Unix platforms as soon as we split up our build
instructions into multiple files.

Fix the issue now to avoid future breakage by including the
"FindThreads" module.

CMakeLists.txt

@@ -24,6 +24,7 @@ INCLUDE(CheckSymbolExists)
 INCLUDE(CheckStructHasMember)
 INCLUDE(AddCFlagIfSupported)
 INCLUDE(FindPkgConfig)
+INCLUDE(FindThreads)
 
 # Build options
 #