I think what is happening is that Mac OS 10.4 comes with a libcurl  
that is too old:

configure says:

checking for libcurl... 7.13.1
checking for curl_global_init in -lcurl... yes

and then make says:

[ghc] src/hscurl.o
src/hscurl.c: In function 'perform':

src/hscurl.c:85:0:
      warning: implicit declaration of function 'curl_multi_timeout'
[ghc] src/rts.o
Linking darcs ...
/usr/bin/ld: Undefined symbols:
_curl_multi_timeout
collect2: ld returned 1 exit status
make: *** [darcs] Error 1

I don't know how to write configure.ac language, but if someone gives  
me some hand-holding I can learn how to teach configure.ac to make a  
configure that checks for curl_multi_timeout().

Regards,

Zooko