I'm not clear on the question but if you don't have the AIX include files, you are going to have trouble trying to develop code that will compile on AIX. AIX often throws in some oddity that needs to be worked around in the include files.
As far as which compiler, I use gcc. Start by pulling down the gcc from IBM: http://www-03.ibm.com/systems/power/software/aix/linux/toolbox/ezinstall.html Then pick a GCC and compile it using the GCC you pulled down. Then use the new GCC for your development. An alternative would be to pull down the GCC from perzl: http://www.perzl.org/aix/index.php?n=Main.Gcc -- I've never tried that but I've heard good things about them.
Hope this helps
Update to Alex:
Yes. As mentioned, the gcc supplied by IBM will give you fits but it is good enough to compile your own gcc which will then be very stable. I've written device drivers using C++ for AIX, used g++, and included parts of Boost.
gcc/g++ for AIX is very stable and complete if compiled properly. Aside from that project, I have about 50 open source packages working on AIX. Most are C and not C++ but in general, if you are doing open source, gcc on AIX will give you less obstacles than xlc. Not xlc's fault. Its just what the developers assume. Like perzl, I use to upload my images as installp install images but had no users so I stopped.
Last: the customary warning is to not use GNU's ld. Use AIX's ld and assembler (as) but you can find threads that will contradict this. I have not seriously tried GNUs ld although I took a stab at it once about 6 months ago and then got side tracked.
Two of my projects you might want to look at:
https://github.com/pedz/aix-build-scripts -- this is a (probably hard to follow) set of scripts to start from ground zero and get you rolling compiling open source projects on AIX.
https://github.com/pedz/aixbin -- This is a set of two scripts that I now have ambivalent feelings about. Many open source projects (Ruby in particular) need these scripts but others (GNU Emacs) will fail if they are used.