optional package in Build-Depends (how?)

[Dmitry Smirnov <onlyjob@member.fsf.org>]

Dear mentors,

I have an interesting problem on my hands:

The package I need to build have optional build dependency (libgpm-dev)
which is not available on all platforms.

If I just put it to Build-Depends, package will FTBFS on some platforms.

So idea is to specify an optional (soft) build-dependency like

    Build-Depends: libgpm-dev | TRUE

where 'TRUE' would stand for essential package like 'dpkg'.

However I suspect there must be a better way to do that, a practical 
equivalent to hypothetical 'Build-Recommends'.

What would be the best practice for such case?
Any suggestions?

Thank you.

All the best,
Dmitry.