acinclude: fix --without-ca-path when cross-compiling

The commit 7b074a460b64811 to CURL_CHECK_CA_BUNDLE in 7.31 (don't check
for paths when cross-compiling) causes --without-ca-path to no longer
works when cross-compiling, since ca and capath only ever get set to
"no" when not cross-compiling, I attach a patch that works for me. Also
in the cross-compilation case, no ca-path seems to be a better default
(IMVHO) than empty ca-path.

Bug: http://curl.haxx.se/bug/view.cgi?id=1273
Patch-by: Stefan Neis