Revert 2101353 for lexical_cast header

`#pragma once` isn't a good fit for this file since it's designed to be parsed multiple times if
LEXICAL_CAST_DEBUG is defined. pragma does not allow that once `once` has been encountered once.

src/lexical_cast.hpp

@@ -21,7 +21,12 @@
  * real job. This is done for the unit tests but should normally not be done.
  */
 
-#pragma once
+#ifdef LEXICAL_CAST_DEBUG
+#undef LEXICAL_CAST_HPP_INCLUDED
+#endif
+
+#ifndef LEXICAL_CAST_HPP_INCLUDED
+#define LEXICAL_CAST_HPP_INCLUDED
 
 #ifdef LEXICAL_CAST_DEBUG
 
@@ -485,3 +490,6 @@ struct lexical_caster<
 };
 
 } // namespace implementation
+
+#endif
+