configure eruby => nil at line 310

I would like to try eRuby on my home machine.   The tarball is unpacked 
and i'm trying to configure:

bash => ./configure.rb --with-apr-includes=/usr/include/apr-1
./configure.rb:310: private method `gsub!' called for nil:NilClass 
(NoMethodError)

What is the problem?  Google didn't provide much help.  The offending 
line and the one before it are:
$XLDFLAGS = CONFIG["XLDFLAGS"]
$XLDFLAGS.gsub!(/-L\./, "")

Thanks,
DSW