Support compilation on aix with gcc.

Shared libraries and the IBM compiler are currently not supported.
"--arch=ppc" has to be passed to configure, uname is not compatible.

configure

@@ -3392,6 +3392,8 @@ enabled spic && enable_weak pic
 
 # OS specific
 case $target_os in
+    aix)
+        ;;
     haiku)
         prefix_default="/boot/common"
         network_extralibs="-lnetwork"