variable_requires
-----------------

Deprecated. Use the if() command instead.

Assert satisfaction of an option's required variables.

::

  variable_requires(TEST_VARIABLE RESULT_VARIABLE
                    REQUIRED_VARIABLE1
                    REQUIRED_VARIABLE2 ...)

The first argument (TEST_VARIABLE) is the name of the variable to be
tested, if that variable is false nothing else is done.  If
TEST_VARIABLE is true, then the next argument (RESULT_VARIABLE) is a
variable that is set to true if all the required variables are set.
The rest of the arguments are variables that must be true or not set
to NOTFOUND to avoid an error.  If any are not true, an error is
reported.
