Change default prefix from /opt to /opt/${PROGRAM}

[aixtools]

I know this is a Linux reference, but I feel the need to comply along these lines.

https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s13.html

And while 'aixtools' is not LANANA , it is IANA registered.

[aixtools]

Notice that LDFLAGS will need an update to include ${prefix}/lib