[Bug #20928] Fix build when malloc_usable_size is available

Copy from gc/default/default.c and revert the part of 51bd816.

Author: nobu

gc.c

@@ -30,6 +30,16 @@
 
 /* MALLOC_HEADERS_BEGIN */
 #ifndef HAVE_MALLOC_USABLE_SIZE
+# ifdef _WIN32
+#  define HAVE_MALLOC_USABLE_SIZE
+#  define malloc_usable_size(a) _msize(a)
+# elif defined HAVE_MALLOC_SIZE
+#  define HAVE_MALLOC_USABLE_SIZE
+#  define malloc_usable_size(a) malloc_size(a)
+# endif
+#endif
+
+#ifdef HAVE_MALLOC_USABLE_SIZE
 # ifdef RUBY_ALTERNATIVE_MALLOC_HEADER
 /* Alternative malloc header is included in ruby/missing.h */
 # elif defined(HAVE_MALLOC_H)
@@ -39,16 +49,6 @@
 # elif defined(HAVE_MALLOC_MALLOC_H)
 #  include <malloc/malloc.h>
 # endif
-
-# ifdef _WIN32
-#  define HAVE_MALLOC_USABLE_SIZE
-#  define malloc_usable_size(a) _msize(a)
-# elif defined HAVE_MALLOC_SIZE
-#  define HAVE_MALLOC_USABLE_SIZE
-#  define malloc_usable_size(a) malloc_size(a)
-# endif
-#else
-# include <malloc.h>
 #endif
 
 /* MALLOC_HEADERS_END */