zz40-xc-ovr.m4: reinstate strict AC_REQUIRE macro dependencies
zz40-xc-ovr.m4: fix variable assignment of subshell output bashism

Tor Arntsen analyzed and reported the issue.

URL: http://curl.haxx.se/mail/lib-2013-01/0306.html
Merge branch 'master' of git://github.com/bagder/curl