I'm getting errors that purelib.rb can't be found compiling Ruby 1.9 
on MacOS 10.5.2.

ruby trunk r16372

mkdir ruby1.9
cd ruby1.9/
svn co http://svn.ruby-lang.org/repos/ruby/trunk src
cd src
autoconf
./configure --prefix=/Users/stephen/dev/ruby1.9
make

...

gcc -g -O2 -pipe -fno-common    -I. -I.ext/include/i686-darwin9.2.2 
-I./include -I.  -DRUBY_EXPORT -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -o 
blockinlining.o -c blockinlining.c
rm -f ruby
ruby -Ks ./tool/insns2vm.rb --srcdir="." insns.inc
ruby: no such file to load -- purelib.rb (LoadError)
make: *** [insns.inc] Error 1

However purelib.rb is in ext/ dir:

$ ls ext/purelib.rb
ext/purelib.rb