yabause fails to cross build from source as it passes host compiler
flags to the build compiler. It already is pretty clever as it has two
code generators that are used as ExternalProject_Add to switch the
compiler from host to build, but Debian communicates compiler flags via
the environment and these ExternalProjects do not yet clear the
environment. As a result, the host compiler flags set up by Debian leak
into those ExternalProjects and trip up the build architecture compiler
there. I'm attaching a patch that clears them. Once applied, yabause
cross builds successfully. What do you think?

Helmut