Change _variable_is such that the name of the variable is passed.

This allows us to check for variables that have a value of an
empty string.

This is an API breaking change and may require some callers of
assert_variable_is and assert_variable_is_not to be changed such
that they pass the variable name and not its value
Merge pull request #34 from smspillaz/cmake-unit.item_32

Change _variable_is such that the name of the variable is passed.