mkmf.rb: fail if cross compiling

* lib/mkmf.rb (MakeMakefile#try_run): bail out explicitly if cross
  compiling, because it cannot work of course.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Fri Feb 15 18:24:48 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/mkmf.rb (MakeMakefile#try_run): bail out explicitly if cross
+	  compiling, because it cannot work of course.
+
 Fri Feb 15 12:34:58 2013  Tanaka Akira  <akr@fsij.org>
 
 	* ext/socket/extconf.rb: test struct sockaddr_storage directly.

lib/mkmf.rb

@@ -811,6 +811,7 @@ SRC
   # Returns true when the executable exits successfully, false when it fails,
   # or nil when preprocessing, compilation or link fails.
   def try_run(src, opt = "", &b)
+    raise "cannot run test program while cross compiling" if CROSS_COMPILING
     if try_link0(src, opt, &b)
       xsystem("./conftest")
     else