Fixed bug 3811 - change HAVE_COPYSIGN to HAVE__COPYSIGN in SDL_config_windows.h

Ozkan Sezer

The patch below changes HAVE_COPYSIGN macro in SDL_config_windows.h to
HAVE__COPYSIGN, so that _copysign() can be used in SDL_stdlib.h which
is available in many more windows-targeting toolchains such as MinGW,
MSVC >= 6, etc, and not just  MSVC >= 2013. SDL_stdlib.c already has a
specific check for HAVE__COPYSIGN, so I believe this is reasonable.

include/SDL_config_windows.h

@@ -142,7 +142,7 @@ typedef unsigned int uintptr_t;
 #define HAVE_SQRTF 1
 #define HAVE_TAN 1
 #define HAVE_TANF 1
-#define HAVE_COPYSIGN 1
+#define HAVE__COPYSIGN 1
 #if defined(_MSC_VER)
 /* These functions were added with the VC++ 2013 C runtime library */
 #if _MSC_VER >= 1800