Avoid training :'s in paths

When the path was empty, path_push_unique would cause a training colon
to be added.  This was a known issue that didn't cause any harm, but
while we're here, we'll clean this up.

configure.ac

@@ -607,7 +607,11 @@ path_remove () {
 path_push_unique () {
   x="$(eval echo \$$1)"
   x="$(path_remove "$x" "$2")"
-  eval export $1="$2:$x"
+  if test -z "$x"; then
+    eval export $1="$2"
+  else
+    eval export $1="$2:$x"
+  fi
 }
 
 # tweak platform specific flags