fixes compiler warnings (#9619)

* compiler warnings: adds SYSTEM to [target_]include_directories to skip warnings originating from dependencies

* compiler warnings: uninitialized/unused variables, missing parenthesis, pragma

* compiler warnings: redundant template type, missing curly braces, pass 0 instead of NULL as int argument

* compiler warnings: removes fclose(fp) where fp==nullptr since fclose() has attribute __nonnull((1))

* compiler warnings: uninitialized variables, missing parentheses, missing curly braces

* compiler warnings: ? as lower precedence than <<

* compiler warnings: unused variable

* compiler warnings: unused result

* compiler warnings: undefined/unused variable

* compiler warnings: uninitialized variable

src/slic3r/GUI/Widgets/Scrollbar.cpp

@@ -1,4 +1,3 @@
-#pragma once
 #include <wx/wxprec.h>
 #ifndef WX_PRECOMP
 #include <wx/wx.h>
@@ -311,4 +310,4 @@ void MyScrollbar::OnMouseWheel(wxMouseEvent &event)
 	}
 	Refresh();
 	Update();
-}
+}