build: Add hyphen to custom build tags

so that ./configure --tag=foo makes a version number like v0.9.5-foo
instead of v0.9.5foo

configure

@@ -495,7 +495,10 @@ def configure_node(o):
   else:
     o['variables']['node_use_perfctr'] = 'false'
 
-  o['variables']['node_tag'] = options.tag or ''
+  if options.tag:
+    o['variables']['node_tag'] = '-' + options.tag
+  else:
+    o['variables']['node_tag'] = ''
 
 
 def configure_libz(o):

src/node_version.h

@@ -27,7 +27,7 @@
 #define NODE_PATCH_VERSION 5
 
 #ifndef NODE_TAG
-#  define NODE_TAG ""
+# define NODE_TAG ""
 #endif
 
 #define NODE_VERSION_IS_RELEASE 0