I have built GPC 20041218 on GCC 3.3.4 on AIX 5.2, and thought it might be useful to document my experiences. I wanted to have the capacity to build 64 bit programs, however only 32 bit libraries are installed for GPC. Following my earlier post http://www.gnu- pascal.de/crystal/gpc/en/mail11115.html Waldek suggested trying build a 64 bit version of GCC including GPC. However having tried various strategies I was not successful in building a 64 bit only version of any version of GCC, even without GPC. So I tried to just build a 64 bit libgpc.a. I eventually achieved this 
export OBJECT_MODE = 64
export CFLAGS=-maix64

then run ./configure in ...gcc3.3.4/gcc/p/rts directory

edit Makefile
add -maix64 to CFLAGS (CFLAGS=-O2)
 
copy libgpc.a to    ...lib/gcc-lib/power..../3.3.4/ppc64

This allowed me generate 64 bit programs. My program contained a reference to the memavail function in  system.pas. I had earlier had some problems with this function in 32 bit mode when it gave an error http://www.gnu-pascal.de/crystal/gpc/en/mail11086.htm

In 64 bit mode the use of this function did not report an error but partly disabled the operating system, which should have been quite impossible for a normal user without root privileges! The operator had to reset the machine following this error. Programs without this function call seem to work fine.