ayatana-indicator-messages fails to cross build from source, because it
uses the build architecture vapigen. This tool actually is
architecture-dependent, because the library search path being used is
architecture-dependent. Therefore, a host architecture vapigen must be
used. It is best discovered from the corresponding vapigen.pc pkg-config
file. I'm attaching a patch for this problem.

Once fixing it, ayatana-indicator-messages continues to fail cross
building. The next problem is the gtk-doc scanner. The typical solution
here is separating the documentation to an Arch:all package and skipping
gtk-doc in arch-only builds. However, I consider this out of scope for
this bug report. Please close it once addressing the vapigen matter
only.

Helmut