> tomc@tom-desktop:/usr/local/src/rubygems-1.2.0$ ruby setup.rb
> ./lib/rubygems/spec_fetcher.rb:1:in `require': no such file to load --
> zlib (LoadError)

I run Ubuntu 8.04.1 with my own compiled ruby 1.8.6p114, which is very 
close to your setup. The extension exists in 
/usr/local/lib/ruby/1.8/i686-linux/zlib.so

Possibly, the package zlib1g-dev wasn't present on your system at the 
time when you compiled ruby, and therefore the Ruby zlib extension 
wasn't built.

If your build directory is still around (i.e. where you untarred the 
source), look in ext/zlib/mkmf.log and see if it points to the problem. 
Here's mine:

have_library: checking for deflateReset() in -lz... -------------------- 
yes

"gcc -o conftest -I../.. -I../../. -I../.././ext/zlib  -g -O2 conftest.c 
-L'.' -L'../..' -L.  -rdynamic -Wl,-export-dynamic     -lruby-static -lz 
-ldl -lcrypt -lm   -lc"
conftest.c: In function ???t???:
conftest.c:3: error: ???deflateReset??? undeclared (first use in this 
function)
conftest.c:3: error: (Each undeclared identifier is reported only once
conftest.c:3: error: for each function it appears in.)
checked program was:
/* begin */
1: /*top*/
2: int main() { return 0; }
3: int t() { void ((*volatile p)()); p = (void ((*)()))deflateReset; 
return 0; }
/* end */

"gcc -o conftest -I../.. -I../../. -I../.././ext/zlib  -g -O2 conftest.c 
-L'.' -L'../..' -L.  -rdynamic -Wl,-export-dynamic     -lruby-static -lz 
-ldl -lcrypt -lm   -lc"
checked program was:
/* begin */
1: /*top*/
2: int main() { return 0; }
3: int t() { deflateReset(); return 0; }
/* end */

--------------------

have_header: checking for zlib.h... -------------------- yes

"gcc -E -I../.. -I../../. -I../.././ext/zlib  -g -O2  conftest.c -o 
conftest.i"
checked program was:
/* begin */
1: #include <zlib.h>
/* end */

--------------------

In any case, the thing to do now is recompile ruby, and carefully look 
at the ./configure output to see if it says why zlib isn't being built. 
It may also be helpful to install libssl-dev first, if you haven't 
already.

HTH,

Brian.
-- 
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