You can clone with
$ gem install rpeg-markdown
Building native extensions. This could take a while...
ERROR: Error installing rpeg-markdown:
ERROR: Failed to build gem native extension.
checking for pkg-config... yes
Makefile:61: *** missing separator. Stop.
Gem files will remain installed in /Users/ujihisa/.gem/ruby/1.9.1/gems/rpeg-markdown-1.4.6 for inspection.
Results logged to /Users/ujihisa/.gem/ruby/1.9.1/gems/rpeg-markdown-1.4.6/ext/gem_make.out
The reason is that the Makefile line #60 is splitted into two lined with a \n; I ascertained that I could build if I fixed the Makefile manually.