Fix build on AIX

[johnsonjh]

@mezantrop

Hi, do you want PRs and patches here, or on SourceForge?

I have a few bugfixes and compatibility changes I'd like to get upstream with you.

First off though, for fixing the build on AIX, the preprocessor definition is _AIX and not __AIX as you have it in the source. A simple global replace fixes it, or building it like env CFLAGS="-D__AIX" gmake LIBS= works as a workaround.

[mezantrop]

Hello @johnsonjh,

Historically Sourceforge is the primary place, so it would be great to initiate changes there. But if it's not possible, I'll be happy to accept them on Github.

Anyway, I appreciate your work and will gladly review them.