acx.m4: Name cache variables properly.

* acx.m4: Name cache variables properly.
(NCN_STRICT_CHECK_TOOL): If program is not found and
value-if-not-found is empty, use ${ncn_tool_prefix}$2 or $2,
depending on whether build != host or not.
(NCN_STRICT_CHECK_TARGET_TOOL): Ditto, with the target prefix.

From-SVN: r60581