element addition in schema based on conditional flag

Hi,

I am new to this list. I am not sure if this sort of question has been
asked earlier.

My requirement is something like this:

#ifdef IS_ADDR_DEFINED
<xsd:element name="addr".....
...................
...................
</xsd:element>
#endif

I want addr element to be included in the schema only if
IS_ADDR_DEFINED is defined. IS_ADDR_DEFINED is to be read from a
makefile.

I am generating #define for each element present in the schema. So I
want to generate #define for addr only if IS_ADDR_DEFINED=1.

How do I do this in XML schema?

Any help would be appreciated.

Thanks,
Neelam