Fix check for --with-ipopt=<nothing here>

[[Imported from SVN: r5146]]

m4/ipopt.m4

@@ -15,7 +15,7 @@ ac_save_PKG_CONFIG_PATH="$PKG_CONFIG_PATH"
 ## do nothing if --without-ipopt is used
 AS_IF([test x$with_ipopt != xno], [
     # is --with-ipopt=bla used?
-    AS_IF([test "x$with_ipopt" != x], [
+    AS_IF([test "x$with_ipopt" != xyes], [
         AS_IF([! test -d $with_ipopt], [
             AC_MSG_WARN([IPOpt directory $with_ipopt does not exist])
         ], [