Add check for madvise(2) to configure.ac.

Some platforms, such as Google's Portable Native Client, use Newlib and
thus lack access to madvise(2).  In those instances, pages_purge() is
transformed into a no-op.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -52,6 +52,11 @@
  */
 #undef JEMALLOC_HAVE_BUILTIN_CLZ
 
+/*
+ * Defined if madvise(2) is available.
+ */
+#undef JEMALLOC_HAVE_MADVISE
+
 /*
  * Defined if OSSpin*() functions are available, as provided by Darwin, and
  * documented in the spinlock(3) manual page.