rpm on AIX (at least 5.x) is old.  RPM 3.3. Ouch.

Anyway, to build the SRPMs here for ruby, you'll need to satisfy the requires.  Normally this involves getting several rpms from the IBM Linux toolkit for AIX, and installing them.  You'll get into a small dependency hell, as yum or apt isn't there for rpm on AIX.  http://www-03.ibm.com/systems/power/software/aix/linux/toolbox/alpha.html

To build ruby you'll need
 openssl-devel, readline-devel, zlib-devel, gcc

On my system I had the following RPMs, (but not all there so I could build ruby)

cdrecord-1.9-7
mkisofs-1.13-4
gtk+-1.2.10-4
autoconf-2.59-1
automake-1.8.5-1
atk-1.4.0-1
bash-3.0-1
bash-doc-3.0-1
binutils-2.14-3
bison-1.875-3
coreutils-5.2.1-2
cpio-2.5-1
diffutils-2.8.1-1
readline-devel-4.3-2
flex-2.5.4a-6
zlib-1.2.2-4
freetype2-2.1.7-2
fontconfig-2.2.2-3
gawk-3.1.3-1
gcc-4.0.0-1
gcc-c++-4.0.0-1
gdb-6.0-1
gettext-0.10.40-1
glib-1.2.10-2
glib2-2.2.3-3
less-382-1
libgcc-4.0.0-1
libjpeg-6b-6
libpng-1.2.1-6
libstdc++-4.0.0-1
libstdc++-devel-4.0.0-1
libtiff-3.6.1-2
m4-1.4.1-1
libtool-1.5.8-1
lsof-4.61-4
make-3.80-1
openssl-0.9.7g-1
openssl-devel-0.9.7g-1
openssl-doc-0.9.7g-1
sudo-1.7.2p5-1
xft-0.0-0
xrender-0.0-0
pango-1.2.5-1
gtk2-2.2.4-4
wu-ftpd-2.6.2-6
ncurses-5.2-3
ncurses-devel-5.2-3
popt-1.7-1
expat-1.95.7-4
zlib-devel-1.2.3-3
wget-1.9.1-1ssl
readline-4.3-2
rsync-3.0.6-1
tar-1.14-2
AIX-rpm-5.3.9.0-8


I also recommend installing bash, because ksh makes me sad.


To build:
rpm -Uvh ruby-1.8.7_p330-1.src.rpm
cd /usr/src/packages/SPECS
rpm -ba ruby.spec

# output should be in /usr/src/packages/ppc

Then to build rubygems, you should just install the ruby 1.8.7 you just built.