ac: Fix AIX shared library builds.

AIX uses ".a" for both static and shared library extensions.
diff --git a/configure.ac b/configure.ac
index 7518976..44fb779 100644
--- a/configure.ac
+++ b/configure.ac
@@ -230,6 +230,8 @@
         LIB_EXTENSION='dylib' ;;
     cygwin* )
         LIB_EXTENSION='dll' ;;
+    aix* )
+        LIB_EXTENSION='a' ;;
     * )
         LIB_EXTENSION='so' ;;
     esac