/tmp/ruby-GD-0.7.4
$ ruby extconf.rb
checking for main() in -lz... yes
checking for main() in -lpng... yes
checking for gdImagePng() in -lgd... yes   
checking for main() in -lm... yes
checking for gdImageColorResolve()... yes
creating Makefile                        
/tmp/ruby-GD-0.7.4
$ make
gcc -g -O2   -I. -I/usr/lib/ruby/1.8/i386-cygwin -I/usr/lib/ruby/1.8/i386-cygwin -I. -DHAVE_GDIMAGECOLORRESOLVE -I/usr/local/include  -c GD.c
gcc -shared -s -Wl,--enable-auto-import,--export-all  -L"/usr/local/lib" -L"/usr/lib" -o GD.so GD.o  -lruby -lm -lgd -lpng -lz  -lcrypt
/tmp/ruby-GD-0.7.4
$ make install
/usr/bin/install -c -m 0755 GD.so /usr/lib/ruby/site_ruby/1.8/i386-cygwin
/tmp/ruby-GD-0.7.4
$ ruby -ve 'require "GD"'
ruby 1.8.3 (2005-09-21) [i386-cygwin]
/usr/lib/ruby/site_ruby/1.8/i386-cygwin/GD.so: No such file or directory - /usr/lib/ruby/site_ruby/1.8/i386-cygwin/GD.so (LoadError)
from -e:1
exit 1

意味分からん。いつかはちゃんと動いていたのになぁ。拡張ライブラリが反抗期で僕は悲しい。