Add DEBUG_LOG_MSG macro

For printing logs only when DEBUG macro is turned on (without sprinkling
idefs all around the code).

include/logging.h

@@ -18,6 +18,9 @@
 
 #ifndef DOSBOX_LOGGING_H
 #define DOSBOX_LOGGING_H
+
+#include <cstdio>
+
 enum LOG_TYPES {
 	LOG_ALL,
 	LOG_VGA, LOG_VGAGFX,LOG_VGAMISC,LOG_INT10,
@@ -86,5 +89,31 @@ void GFX_ShowMsg(char const* format,...) GCC_ATTRIBUTE(__format__(__printf__, 1,
 
 #endif //C_DEBUG
 
+#ifdef NDEBUG
+// DEBUG_LOG_MSG exists only for messages useful during development, and not to
+// be redirected into internal DOSBox debugger for DOS programs (C_DEBUG feature).
+#define DEBUG_LOG_MSG(...)
+#else
+// There's no portable way to expand variadic macro using C99/C++11 (or older)
+// alone. This language limitation got removed only with C++20 (through addition
+// of __VA_OPT__ macro).
+#ifdef _MSC_VER
+#define DEBUG_LOG_MSG(fmt, ...) fprintf(stderr, fmt "\n", __VA_ARGS__)
+//                                                      ~
+// MSVC silently eliminates trailing comma if needed:   ^
+#else
+#define DEBUG_LOG_MSG(fmt, ...) fprintf(stderr, fmt "\n", ##__VA_ARGS__)
+//                                                        ~~~~~~~~~~~~~
+//                                                        ^
+// GCC and Clang provide '##' token as a language extension to explicitly remove
+// trailing comma when there's no trailing parameters.
+#endif
+// If it'll ever be necessary to support another compiler, which does not
+// support C++20 nor GNU extension nor MSVC extension, then behaviour can be
+// simulated by implementing C++ variadic template wrapped inside a macro
+// (at the cost of slowing down compilation).
+//
+// Another option it to use vsnprintf (at the cost of slowing down runtime).
+#endif // NDEBUG
 
-#endif //DOSBOX_LOGGING_H
+#endif