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.

configure.ac

@@ -237,6 +237,7 @@ dnl Define cpp macros in CPPFLAGS, rather than doing AC_DEFINE(macro), since the
 dnl definitions need to be seen before any headers are included, which is a pain
 dnl to make happen otherwise.
 default_munmap="1"
+JEMALLOC_USABLE_SIZE_CONST="const"
 case "${host}" in
   *-*-darwin*)
 	CFLAGS="$CFLAGS"
@@ -262,6 +263,7 @@ case "${host}" in
 	abi="elf"
 	AC_DEFINE([JEMALLOC_PURGE_MADVISE_DONTNEED], [ ])
 	AC_DEFINE([JEMALLOC_THREADED_INIT], [ ])
+	JEMALLOC_USABLE_SIZE_CONST=""
 	default_munmap="0"
 	;;
   *-*-netbsd*)
@@ -323,6 +325,7 @@ case "${host}" in
 	abi="elf"
 	;;
 esac
+AC_DEFINE_UNQUOTED([JEMALLOC_USABLE_SIZE_CONST], [$JEMALLOC_USABLE_SIZE_CONST])
 AC_SUBST([abi])
 AC_SUBST([RPATH])
 AC_SUBST([LD_PRELOAD_VAR])