Correction: copysign has been supported by windows several toolchains

for a very long time, including MSVC6, MinGW, LCC-Win32, (no released
watcom versions though, but that's of no concern.)

Patch from Ozkan Sezer

include/SDL_config_windows.h

@@ -142,12 +142,12 @@ typedef unsigned int uintptr_t;
 #define HAVE_SQRTF 1
 #define HAVE_TAN 1
 #define HAVE_TANF 1
+#define HAVE_COPYSIGN 1
 #if defined(_MSC_VER)
 /* These functions were added with the VC++ 2013 C runtime library */
 #if _MSC_VER >= 1800
 #define HAVE_STRTOLL 1
 #define HAVE_VSSCANF 1
-#define HAVE_COPYSIGN 1
 #define HAVE_SCALBN 1
 #endif
 /* This function is available with at least the VC++ 2008 C runtime library */