Add --with-malloc-conf.

Add --with-malloc-conf, which makes it possible to embed a default
options string during configuration.
diff --git a/INSTALL b/INSTALL
index 8d39687..5c25054 100644
--- a/INSTALL
+++ b/INSTALL
@@ -84,6 +84,14 @@
     versions of jemalloc can coexist in the same installation directory.  For
     example, libjemalloc.so.0 becomes libjemalloc<suffix>.so.0.
 
+--with-malloc-conf=<malloc_conf>
+    Embed <malloc_conf> as a run-time options string that is processed prior to
+    the malloc_conf global variable, the /etc/malloc.conf symlink, and the
+    MALLOC_CONF environment variable.  For example, to change the default chunk
+    size to 256 KiB:
+
+      --with-malloc-conf=lg_chunk:18
+
 --disable-cc-silence
     Disable code that silences non-useful compiler warnings.  This is mainly
     useful during development when auditing the set of warnings that are being