COMP: Fix vxl_config_macros checks broken by refactoring

In commit f910d5e7 (Use if(DEFINED) to simplify conditions, 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.

Change-Id: I15a2a5a583c97ec67aeca9e1f3b20becb396c641
Merge topic 'simplify-if-conditions'

58b65ee3 COMP: Fix vxl_config_macros checks broken by refactoring