Build: Add support for --no-po4a option to autogen.sh.

Normally, if po4a isn't available, autogen.sh will return
with non-zero exit status. The option --no-po4a can be useful
when one knows that po4a isn't available but wants autogen.sh
to still return with zero exit status.

autogen.sh

@@ -21,4 +21,13 @@
 
 # Generate the translated man pages if the "po4a" tool is available.
 # This is *NOT* done by "autoreconf -fi" or when "make" is run.
-cd po4a && sh update-po
+#
+# Pass --no-po4a to this script to skip this step. It can be useful when
+# you know that po4a isn't available and don't want autogen.sh to exit
+# with non-zero exit status.
+if test "x$1" != "x--no-po4a"; then
+	cd po4a
+	sh update-po
+fi
+
+exit 0