Passing a block to methods in mkmf.rb

Hi,

I'm curious as to what the optional block is for in the mkmf.rb methods
like have_header, etc.  I thought it merely yielded back the C source
that it was trying to compile behind the scenes to do the check.
However, when I try to pass a block I get errors:

# extconf.rb test
have_header("windows.h"){ |code| p code }

On Windows XP Pro:

C:\eclipse\workspace\ruby-foo>ruby extconftest.rb
checking for windows.h... c:/ruby/lib/ruby/1.8/fileutils.rb:687:in
`unlink': Per
mission denied - mkmftmp.log (Errno::EACCES)
        from c:/ruby/lib/ruby/1.8/fileutils.rb:687:in `remove_file'
        from c:/ruby/lib/ruby/1.8/fileutils.rb:618:in `rm'
        from c:/ruby/lib/ruby/1.8/fileutils.rb:617:in `each'
        from c:/ruby/lib/ruby/1.8/fileutils.rb:617:in `rm'
        from c:/ruby/lib/ruby/1.8/fileutils.rb:634:in `rm_f'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:109:in `rm_f'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:177:in `postpone'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:168:in `open'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:168:in `postpone'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:467:in `checking_for'
        from c:/ruby/lib/ruby/1.8/mkmf.rb:528:in `have_header'
        from extconftest.rb:3

On Solaris 9:

djberge@sp5wd-b1-/home/djberge/programming/ruby-534>ruby extconftest.rb
checking for wait.h... /opt/lib/ruby/1.8/mkmf.rb:215:in `create_tmpsrc':
private
 method `sub' called for nil:NilClass (NoMethodError)
        from /opt/lib/ruby/1.8/mkmf.rb:223:in `try_do'
        from /opt/lib/ruby/1.8/mkmf.rb:276:in `try_cpp'
        from /opt/lib/ruby/1.8/mkmf.rb:529:in `have_header'
        from /opt/lib/ruby/1.8/mkmf.rb:528:in `checking_for'
        from /opt/lib/ruby/1.8/mkmf.rb:467:in `postpone'
        from /opt/lib/ruby/1.8/mkmf.rb:172:in `open'
        from /opt/lib/ruby/1.8/mkmf.rb:172:in `postpone'
        from /opt/lib/ruby/1.8/mkmf.rb:168:in `open'
        from /opt/lib/ruby/1.8/mkmf.rb:168:in `postpone'
        from /opt/lib/ruby/1.8/mkmf.rb:467:in `checking_for'
        from /opt/lib/ruby/1.8/mkmf.rb:528:in `have_header'
        from extconftest.rb:3

If that's not what the block is for, then what is it for?  And can
someone provide an example?

Or is this a bug?

Regards,

Dan