It would appear your installers didn't install one. I don't think recent versions of AIX come with one as a default, but there is a crippled version of cc you can install from the CDs.
What you need for LAWSON purposes is the COMMERCIAL AIX compiler, that you have to pay for - separate and above the OS. You may need that one to compile Apache as well. So you should probably look to your IBM distributor and purchase the latest version supported by your OS.
I can save you the heartache of compiling the mod_jk.so. You can pick up a pre-compiled mod_jk.so from my site,
http://www.schronce.net/lawson