助田です。
Cygwinまたは私の環境の問題のような気がするのですが一応報告しておきます。

trunkの話です。
Cygwin上で CC='gcc -mno-cygwin'でmingw32版Rubyをコンパイルしようとする
とErrorになります。

gcc -mno-cygwin -O2 -g -Wall -Wno-parentheses   -I.
-I.ext/include/i386-mingw32 -I../../../../svn/ruby/trunk/include
-I../../../../svn/ruby/trunk  -DRUBY_EXPORT   -o main.o -c
../../../../svn/ruby/trunk/main.c
In file included from ../../../../svn/ruby/trunk/include/ruby/ruby.h:68,
                 from ../../../../svn/ruby/trunk/include/ruby.h:33,
                 from ../../../../svn/ruby/trunk/main.c:13:
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:
In function `fprintf':
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:245:
error: conflicting types for '__p___argv'
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdlib.h:78:
error: previous declaration of '__p___argv' was here
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:
In function `printf':
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:255:
error: conflicting types for '__p___argv'
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdlib.h:78:
error: previous declaration of '__p___argv' was here
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:
In function `sprintf':
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdio.h:265:
error: conflicting types for '__p___argv'
/usr/lib/gcc/i686-pc-mingw32/3.4.4/../../../../i686-pc-mingw32/include/stdlib.h:78:
error: previous declaration of '__p___argv' was here
make: *** [main.o] Error 1

Cygwinのmingw32-runtime を3.15-1から3.14-1にダウングレードすると
コンパイルできました。

  助田 雅紀