I was able to get the 64 bit version of QuickFIX library to compile on AIX 6100-09-05-1524 with IBM XL C/C++ for AIX, V12.1.
Unit tests do not compile so these were disabled.

Also, for building please refer to
https://www.ibm.com/support/knowledgecenter/SSGH3R_13.1.0/com.ibm.xlcpp131.aix.doc/proguide/threadsafe_streams.html
as this may have an impact on your application (like examples).

Example script:

#!/bin/sh

cflagx="-q64 -D_THREAD_SAFE -qthreaded"
cxxflagx="-q64 -D_THREAD_SAFE -qthreaded -D__IBMCPP_TR1__=1"
ldflagx="-q64 -D_THREAD_SAFE -qthreaded"

OBJECT_MODE=64
export OBJECT_MODE

./bootstrap

CC=xlc_r CXX=xlC_r CFLAGS=$cflagx CXXFLAGS=$cxxflagx LDFLAGS=$ldflagx configure --prefix="Your prefix"

make && make install