AIX systems need to use the SYSV stty(1) variant.

In generating CSHENV prompts, I used stty(1) to find the width
of the current terminal, at sourcing time.  SYSV and BSD use
different output formats.  Presently I have tcshrc.prompt.defs
default to 80 columns when I don't know how to parse stty(1)'s
output.  AIX is known to be a SYSV variant, and I've added a
case to handle this.

tcshrc.prompt.defs

@@ -58,6 +58,7 @@ switch( $CSHENV_SYSTEM )
 	case Linux:
 
     # SYSV type systems seem to be settling on this as the common mechanism.
+    case AIX:
     case SunOS:
 		if( $?CSHENV_DEBUG ) then
 			echo "Linux prompt width for stty blows..."