Change grfmsg to be a macro which checks the debug level first

This is to avoid doing a sprintf when the result will be thrown away

src/newgrf.cpp

@@ -377,7 +377,7 @@ static GRFLineToSpriteOverride _grf_line_to_action6_sprite_override;
  * @param severity debugging severity level, see debug.h
  * @param str message in printf() format
  */
-void CDECL grfmsg(int severity, const char *str, ...)
+void CDECL _intl_grfmsg(int severity, const char *str, ...)
 {
 	char buf[1024];
 	va_list va;

src/newgrf.h

@@ -15,6 +15,7 @@
 #include "cargotype.h"
 #include "rail_type.h"
 #include "fileio_type.h"
+#include "debug.h"
 #include "core/bitmath_func.hpp"
 #include "core/alloc_type.hpp"
 #include "core/smallvec_type.hpp"
@@ -188,7 +189,8 @@ void ReloadNewGRFData(); // in saveload/afterload.cpp
 void ResetNewGRFData();
 void ResetPersistentNewGRFData();
 
-void CDECL grfmsg(int severity, const char *str, ...) WARN_FORMAT(2, 3);
+#define grfmsg(severity, ...) if ((severity) == 0 || _debug_grf_level >= (severity)) _intl_grfmsg(severity, __VA_ARGS__)
+void CDECL _intl_grfmsg(int severity, const char *str, ...) WARN_FORMAT(2, 3);
 
 bool GetGlobalVariable(byte param, uint32 *value, const GRFFile *grffile);
 

src/newgrf_class_func.h

@@ -9,6 +9,7 @@
 
 /** @file newgrf_class_func.h Implementation of the NewGRF class' functions. */
 
+#include "newgrf.h"
 #include "newgrf_class.h"
 
 #include "table/strings.h"