Fix vxl_config_macros usage of CMake check macros

Some CMake check macros used the idiom

  if("${VAR}" MATCHES "^${VAR}$")

to detect whether a variable is set.  In vxl_config_macros we were
abusing this implementation by using

  SET( ${VARIABLE} ${VARIABLE} )

to reset the check state.  This prevents the CMake macros from using

  if(NOT DEFINED "${VAR}")

instead of the old idiom.  Use the much cleaner

  UNSET(${VARIABLE})

instead.