Rake task only looks for apxs executable; however, on my install ( Ubuntu server 7.10 ) I had only a apxs2 executable. Therefore, I had to make a symbolic link for apxs.
I added ability to pass in location of apxs (or apxs2). For example,
%> rake APXS=/usr/local/apache2/bin/apxs
See my fork. Hopefully, it can be merged into the original.