File.basename() and mkmf.rb

When create_makefile() in mkmf.rb builds an array of targets $objs it 
uses the following code:
     801   unless $objs then
     802     $objs = []
     803     for f in Dir[File.join(srcdir, 
"*.{#{SRC_EXT.join(%q{,})}}")]
     804       $objs.push(File.basename(f, ".*") << "." << $OBJEXT)
     805     end
     806   else
     807     for i in $objs
     808       i.sub!(/\.o\z/, ".#{$OBJEXT}")
     809     end
     810   end
     811   $objs = $objs.join(" ")
My problem is that I have files with multiple extensions (ie. 
'file.y.c').  I have added rules for these files in ./depends and 
changed 'SRC_EXT', but the line:
     804       $objs.push(File.basename(f, ".*") << "." << $OBJEXT)
doesn't do what I want so I just generate $objs myself.
File.basename(f, ".*") only gets rid of the first suffix, not both or 
all suffixes.  Is this by design?
If it is here is the code I use to generate my targets:
###
$objs = []
# create a pattern to match the file extentions in SRC_EXT
ext_pattern = ".(#{SRC_EXT.join('|')})\\z".gsub(/\./, '\.')
ext_regexp = Regexp.new(ext_pattern)
for f in Dir["*.{#{SRC_EXT.join(%q{,})}}"]
   $objs.push(f.gsub(ext_regexp, ".#{$OBJEXT}"))
end
###
Any interest in using something like this to generate targets?
-Charlie