Got this to work so thought I'd let other folk know.
AIX with ssl has a library libssl.a in /usr/linux/lib the configure script is expecting libssl.o. You could put a link file in, but you'd have to do that on every box. So edit the configure script and change libssl.o to libssl.a. Then run configure with the following parameters --with-ssl-inc=/usr/linux/include --with-ssl-lib=/usr/linux/lib
Next make all failed with an undeclared LOG_FTP couldn't work out what this was trying to do, so I edited the src/nrpe.c and commented out the else if and following line.