CMakeLists.txt: detect and use -pthread compiler flag

The cmake thread detection just ain't enough to always work

CMakeLists.txt

@@ -428,6 +428,8 @@ if (UNIX AND NOT APPLE)
   # Note that at the time of this writing the -Wstrict-prototypes flag added below will make this fail
   set(THREADS_PREFER_PTHREAD_FLAG ON)
   find_package(Threads)
+  add_c_flag_if_supported(-pthread CMAKE_C_FLAGS)
+  add_cxx_flag_if_supported(-pthread CMAKE_CXX_FLAGS)
 endif()
 
 # Handle OpenSSL, used for sha256sum on binary updates and light wallet ssl http