Define MADV_FREE on our own when needed.

On x86 Linux, we define our own MADV_FREE if madvise(2) is available, but no
MADV_FREE is detected.  This allows the feature to be built in and enabled with
runtime detection.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -285,6 +285,9 @@
 #undef JEMALLOC_PURGE_MADVISE_DONTNEED
 #undef JEMALLOC_PURGE_MADVISE_DONTNEED_ZEROS
 
+/* Defined if madvise(2) is available but MADV_FREE is not (x86 Linux only). */
+#undef JEMALLOC_DEFINE_MADVISE_FREE
+
 /*
  * Defined if transparent huge pages (THPs) are supported via the
  * MADV_[NO]HUGEPAGE arguments to madvise(2), and THP support is enabled.

include/jemalloc/internal/jemalloc_preamble.h.in

@@ -47,6 +47,10 @@
 #endif
 #include "jemalloc/internal/hooks.h"
 
+#ifdef JEMALLOC_DEFINE_MADVISE_FREE
+#  define JEMALLOC_MADV_FREE 8
+#endif
+
 static const bool config_debug =
 #ifdef JEMALLOC_DEBUG
     true