Fix vxl_config_macros checks broken by refactoring

In commit 9a27bf34 (Simplify vxl_config_macros conditions with
if(DEFINED), 2014-09-08) we removed sevaral uses of implicit variable
evaluation.  However, some of the vxl_config_macros were abusing details
of the prior implementation and depended on setting a check result
variable to its own name to reset the check.  Use unset() instead.

Also fix PERFORM_CHECK_HEADER to correctly detect when the variable
value is empty, rather than its name.  This was also broken by the
previous commit.