If the receipt defines custom properties in the package_info() method
then these properties should appear as make variables, too. The pattern
of the make variable is

CONAN_PROPERTY_{dependency_name}_{property_name}

Furthermore handle case when _properties is None.

- _makefy_properties() always returns a dict
- Simplification of define_multiple_variable_value jinja2 macro

Co-authored-by: James <[email protected]>

It would break the makefile if we would create property values with
newlines. Therefore we output a warning message and skip such
properties.