when i had to install ruby-postgres 0.7.1 within standard ruby on MacOS
X (1.6.8) i had to change lines in "extconf.rb" :

$CFLAGS = " -I/usr/include/openssl "
$LDFLAGS = " -L/usr/lib -lssl -lcrypto "

but, with Ruby 1.8.1 i still have this error :

checking for PQsetdbLogin() in -lpq... no
Could not find PostgreSQL libraries: Makefile not created

i'm sure i'm using ruby 1.8.1 with "extconf.rb"

-- 
yt