Fix another "out of tree build" problem
for gen_test_char when using an old libtool.

Backport of r1578420 from trunk.
No CHANGES needed, because similar issue already
added for 1.5.1.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1578425 13f79535-47bb-0310-9956-ffa450edef68