lib/mkmf.rb: revert last change

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@14665 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -152,7 +152,7 @@ if not $extmk and File.exist?(($hdrdir = RbConfig::CONFIG["rubyhdrdir"]) + "/rub
   $top_srcdir = $hdrdir
   $arch_hdrdir = $hdrdir + "/$(arch)"
 elsif File.exist?(($hdrdir = ($top_srcdir ||= topdir) + "/include")  + "/ruby.h") and
-    File.exist?("#{$top_srcdir}/#{CONFIG["EXTOUT"]}/include/#{CONFIG["arch"]}/ruby/config.h")
+    File.exist?("#{CONFIG["EXTOUT"]}/include/#{CONFIG["arch"]}/ruby/config.h")
   $topdir ||= RbConfig::CONFIG["topdir"]
   $arch_hdrdir = "$(extout)/include/$(arch)"
 else