Build Vim with ruby support but alawys disable

Revision 2
I do config with this:
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>./configure --prefix=/usr/local --enable-gui=no --without-x --disable-nls --enable-multibyte --with-tlib=ncurses --enable-pythoninterp --enable-rubyinterp --with-ruby-command=/usr/bin/ruby --with-features=huge

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and here is my os info:

[samuel@HP6520s src]$ ruby -v
ruby 1.8.7 (2011-06-30 patchlevel 352) [i386-linux]
[samuel@HP6520s src]$ which ruby
/usr/bin/ruby

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but after i config the makefile , it show the following:

67 checking --enable-rubyinterp argument... yes
68 checking --with-ruby-command argument... /usr/bin/ruby
69 checking for /usr/bin/ruby... no    //--------why no???

,
and after i make install vim , it don't support ruby.
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who can tell me why ??? thanks a lot.
:)

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