On windows, can't build outside of srcdir without ruby
nmake does not recognize $(srcdir)/miniprelude.c and so on.
Updated by yugui (Yuki Sonoda) about 12 years ago
ko1 encountered this issue. He renamed the installed ruby to another name so that it would not be used as BASERUBY, and expand the tar.bz2 archive, and built ruby in another directory.
nmake did not recognize $(srcdir)/miniprelude.c, so it tried to generate ./miniprelude.c, and failed because it has no BASERUBY.