make_encmake.rb: expand srcdir

* enc/make_encmake.rb: @srcdir@ in enc/Makefile.in needs to be
  expanded.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51770 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

enc/make_encmake.rb

@@ -9,6 +9,7 @@ if $".grep(/mkmf/).empty?
 end
 require 'erb'
 
+CONFIG['srcdir'] = RbConfig::CONFIG['srcdir']
 CONFIG["MAKEDIRS"] ||= '$(MINIRUBY) -run -e mkdir -- -p'
 
 BUILTIN_ENCS = []