Drop const from malloc_usable_size() argument on Linux.

Drop const from malloc_usable_size() argument on Linux, in order to
match the prototype in Linux's malloc.h.

include/jemalloc/jemalloc.h.in

@@ -59,7 +59,8 @@ JEMALLOC_EXPORT void *	je_memalign(size_t alignment, size_t size)
 JEMALLOC_EXPORT void *	je_valloc(size_t size) JEMALLOC_ATTR(malloc);
 #endif
 
-JEMALLOC_EXPORT size_t	je_malloc_usable_size(const void *ptr);
+JEMALLOC_EXPORT size_t	je_malloc_usable_size(
+    JEMALLOC_USABLE_SIZE_CONST void *ptr);
 JEMALLOC_EXPORT void	je_malloc_stats_print(void (*write_cb)(void *,
     const char *), void *je_cbopaque, const char *opts);
 JEMALLOC_EXPORT int	je_mallctl(const char *name, void *oldp,

include/jemalloc/jemalloc_defs.h.in

@@ -221,6 +221,15 @@
 #undef JEMALLOC_OVERRIDE_MEMALIGN
 #undef JEMALLOC_OVERRIDE_VALLOC
 
+/*
+ * At least Linux omits the "const" in:
+ *
+ *   size_t malloc_usable_size(const void *ptr);
+ *
+ * Match the operating system's prototype.
+ */
+#undef JEMALLOC_USABLE_SIZE_CONST
+
 /*
  * Darwin (OS X) uses zones to work around Mach-O symbol override shortcomings.
  */