Dynamic modules: expose the "modules" target.

auto/install

diff --git a/auto/install b/auto/install
index 9469a49..36226ad 100644
--- a/auto/install
+++ b/auto/install
@@ -190,6 +190,9 @@
 install:
 	\$(MAKE) -f $NGX_MAKEFILE install
 
+modules:
+	\$(MAKE) -f $NGX_MAKEFILE modules
+
 upgrade:
 	$NGX_SBIN_PATH -t