mkmf.rb: strip destdir from prefix

* lib/mkmf.rb (configuration): strip destdir part from prefix to get
  rid of duplication.  a patch by arton at [ruby-core:58859].
  [ruby-core:58856] [Bug #9213]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44019 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,9 @@
+Thu Dec  5 22:18:01 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/mkmf.rb (configuration): strip destdir part from prefix to get
+	  rid of duplication.  a patch by arton at [ruby-core:58859].
+	  [ruby-core:58856] [Bug #9213]
+
 Thu Dec  5 21:53:29 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* array.c (rb_ary_or): lhs elements are prefered, so should not

lib/mkmf.rb

@@ -1849,6 +1849,7 @@ VPATH = #{vpath.join(CONFIG['PATH_SEPARATOR'])}
     prefix = mkintpath(CONFIG["prefix"])
     if destdir = prefix[$dest_prefix_pattern, 1]
       mk << "\nDESTDIR = #{destdir}\n"
+      prefix = prefix[destdir.size..-1]
     end
     mk << "prefix = #{with_destdir(prefix).unspace}\n"
     CONFIG.each do |key, var|