FindProtobuf: also find pthread

Modify FindProtobuf.cmake to find the pthread library on UNIX platforms,
and to add the same to PROTOBUF_LIBRARIES, as this is a link dependency
of libraries using the protobuf headers.

Modules/FindProtobuf.cmake

@@ -156,6 +156,16 @@ function(_protobuf_find_libraries name filename)
    endif()
 endfunction()
 
+# Internal function: find threads library
+function(_protobuf_find_threads)
+    set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
+    find_package(Threads)
+    if(Threads_FOUND)
+        list(APPEND PROTOBUF_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+        set(PROTOBUF_LIBRARIES "${PROTOBUF_LIBRARIES}" PARENT_SCOPE)
+    endif()
+endfunction()
+
 #
 # Main.
 #
@@ -190,6 +200,9 @@ if(MSVC)
     set(CMAKE_FIND_LIBRARY_PREFIXES "${PROTOBUF_ORIG_FIND_LIBRARY_PREFIXES}")
 endif()
 
+if(UNIX)
+    _protobuf_find_threads()
+endif()
 
 # Find the include directory
 find_path(PROTOBUF_INCLUDE_DIR