Fix SYNC_QUEUE build issue on Boost < 1.56

Prior to Boost 1.56, boost::optional had no move constructor, preventing
use of std::unique_ptr inside it

include/footprint_info.h

@@ -235,12 +235,10 @@ public:
 
     std::unique_ptr<IO_ERROR> PopError()
     {
-        auto item = m_errors.pop();
+        std::unique_ptr<IO_ERROR> error;
 
-        if( item )
-            return std::move( *item );
-        else
-            return std::unique_ptr<IO_ERROR>();
+        m_errors.pop( error );
+        return error;
     }
 
     /**

include/sync_queue.h

@@ -20,7 +20,6 @@
 #ifndef SYNC_QUEUE_H
 #define SYNC_QUEUE_H
 
-#include <boost/optional.hpp>
 #include <mutex>
 #include <queue>
 
@@ -59,22 +58,24 @@ public:
     }
 
     /**
-     * Pop a value off the queue if there is one, returning it. If the queue is empty,
-     * return boost::none instead.
+     * Pop a value off the queue into the provided variable. If the queue is empty, the
+     * variable is not touched.
+     *
+     * @return true iff a value was popped.
      */
-    boost::optional<T> pop()
+    bool pop( T& aReceiver )
     {
         GUARD guard( m_mutex );
 
         if( m_queue.empty() )
         {
-            return boost::none;
+            return false;
         }
         else
         {
-            T val = std::move( m_queue.front() );
+            aReceiver = std::move( m_queue.front() );
             m_queue.pop();
-            return std::move( val );
+            return true;
         }
     }
 

pcbnew/footprint_info_impl.cpp

@@ -98,11 +98,13 @@ bool FOOTPRINT_LIST_IMPL::CatchErrors( std::function<void()> aFunc )
 
 void FOOTPRINT_LIST_IMPL::loader_job()
 {
-    while( auto const nickname = m_queue_in.pop() )
+    wxString nickname;
+
+    while( m_queue_in.pop( nickname ) )
     {
         CatchErrors( [this, &nickname]() {
-            m_lib_table->PrefetchLib( *nickname );
-            m_queue_out.push( *nickname );
+            m_lib_table->PrefetchLib( nickname );
+            m_queue_out.push( nickname );
         } );
 
         m_count_finished.fetch_add( 1 );
@@ -183,14 +185,16 @@ bool FOOTPRINT_LIST_IMPL::JoinWorkers()
     for( size_t i = 0; i < std::thread::hardware_concurrency() + 1; ++i )
     {
         threads.push_back( std::thread( [this, &queue_parsed]() {
-            while( auto nickname = this->m_queue_out.pop() )
+            wxString nickname;
+
+            while( this->m_queue_out.pop( nickname ) )
             {
                 CatchErrors( [this, &queue_parsed, &nickname]() {
-                    wxArrayString fpnames = this->m_lib_table->FootprintEnumerate( *nickname );
+                    wxArrayString fpnames = this->m_lib_table->FootprintEnumerate( nickname );
 
                     for( auto const& fpname : fpnames )
                     {
-                        FOOTPRINT_INFO* fpinfo = new FOOTPRINT_INFO_IMPL( this, *nickname, fpname );
+                        FOOTPRINT_INFO* fpinfo = new FOOTPRINT_INFO_IMPL( this, nickname, fpname );
                         queue_parsed.move_push( std::unique_ptr<FOOTPRINT_INFO>( fpinfo ) );
                     }
                 } );
@@ -201,8 +205,10 @@ bool FOOTPRINT_LIST_IMPL::JoinWorkers()
     for( auto& thr : threads )
         thr.join();
 
-    while( auto fpi = queue_parsed.pop() )
-        m_list.push_back( std::move( *fpi ) );
+    std::unique_ptr<FOOTPRINT_INFO> fpi;
+
+    while( queue_parsed.pop( fpi ) )
+        m_list.push_back( std::move( fpi ) );
 
     std::sort( m_list.begin(), m_list.end(),
             []( std::unique_ptr<FOOTPRINT_INFO> const&     lhs,