Fix riscv64 build failure by linking correctly with pthread

Link with -pthread instead of -lpthread, and do that for all architectures.

Contributors:
Aurelien Jarno <aurelien@aurel32.net>

Closes: #1717

configure.cmake

@@ -134,6 +134,7 @@ IF(UNIX)
   IF(NOT LIBRT)
     MY_SEARCH_LIBS(clock_gettime rt LIBRT)
   ENDIF()
+  set(THREADS_PREFER_PTHREAD_FLAG ON)
   FIND_PACKAGE(Threads)
 
   SET(CMAKE_REQUIRED_LIBRARIES