Fix couple of build issues on gcc

src/libslic3r/AppConfig.cpp

@@ -78,9 +78,9 @@ void AppConfig::set_defaults()
             set("single_instance", 
 #ifdef __APPLE__
                 "1"
-#else __APPLE__
+#else // __APPLE__
                 "0"
-#endif __APPLE__
+#endif // __APPLE__
                 );
 
         if (get("remember_output_path").empty())

src/libslic3r/Thread.cpp

@@ -11,6 +11,7 @@
 #include <mutex>
 #include <tbb/parallel_for.h>
 #include <tbb/tbb_thread.h>
+#include <tbb/task_scheduler_init.h>
 
 #define SLIC3R_THREAD_NAME_WIN32_MODERN
 

src/slic3r/GUI/GUI_Init.cpp

@@ -14,6 +14,7 @@
 #include <wx/msgdlg.h>
 
 #include <boost/nowide/iostream.hpp>
+#include <boost/nowide/convert.hpp>
 
 namespace Slic3r {
 namespace GUI {

src/slic3r/GUI/GUI_Init.hpp

@@ -1,7 +1,10 @@
 #ifndef slic3r_GUI_Init_hpp_
 #define slic3r_GUI_Init_hpp_
 
+#include <libslic3r/PrintConfig.hpp>
+
 namespace Slic3r {
+
 namespace GUI {
 
 struct GUI_InitParams
@@ -21,4 +24,4 @@ int GUI_Run(GUI_InitParams &params);
 } // namespace GUI
 } // namespace Slic3r
 
-#endif slic3r_GUI_Init_hpp_
+#endif // slic3r_GUI_Init_hpp_

src/slic3r/GUI/MainFrame.hpp

@@ -9,7 +9,7 @@
 #include <wx/filehistory.h>
 #ifdef __APPLE__
 #include <wx/taskbar.h>
-#endif __APPLE__
+#endif // __APPLE__
 
 #include <string>
 #include <map>