Refactor common PThreads CMake code

Move the flags and find of Threads to root CMakeLists.txt, rather
than duplicate these everywhere. Make explicit linking of library with
PThreads use the same mechanism.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

programs/x509/CMakeLists.txt

@@ -1,7 +1,3 @@
-set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
-set(THREADS_PREFER_PTHREAD_FLAG TRUE)
-find_package(Threads)
-
 set(libs
     ${mbedx509_target}
 )