With mingw and the current 1.9 "nightly snapshot"  I currently get

...
gcc -O2 -g -Wall -Wno-parentheses   -I. -I.ext/include/i386-mingw32
-I./include -I.  -DRUBY_EXPORT   -o parse.o -c parse.c
parse.y:22:19: parse.h: No such file or directory

from running ./configure && make in the newly unzipped directory.
Deleting parse.c from the directory forces parse.y to be rebuilt and
it works after that point--tests pass.

Might want to look into it :)

Thanks!
-=R