Disable the Mingw C runtime by default

Mingw seems to have a bad pow implementation in the C runtime:
'Pow(-72.300000,12.000000), expected [20401381050275984310272.000000], got 20401381050275996893184.000000': Failed

CMakeLists.txt

@@ -154,6 +154,9 @@ endif()
 set(SDL_LIBC_DEFAULT ON)
 set(SDL_SYSTEM_ICONV_DEFAULT ON)
 if(WINDOWS)
+  if(MINGW)
+    set(SDL_LIBC_DEFAULT OFF)
+  endif()
   set(SDL_SYSTEM_ICONV_DEFAULT OFF)
 endif()