cmake: define HAVE_MALLOC_H when malloc.h is found

CMakeLists.txt

@@ -937,7 +937,7 @@ endif()
 set(SDL_DISABLE_ALLOCA 0)
 check_include_file("alloca.h" "HAVE_ALLOCA_H")
 if(MSVC)
-  check_include_file("malloc.h" "HAVE_MALLOC")
+  check_include_file("malloc.h" "HAVE_MALLOC_H")
   check_symbol_exists("_alloca" "malloc.h" _ALLOCA_IN_MALLOC_H)
   if(NOT HAVE_ALLOCA_H AND NOT _ALLOCA_IN_MALLOC_H)
     set(SDL_DISABLE_ALLOCA 1)