As it turns out, the pyAMI build depends on AthPyFwdBwdPort's distribute

installation. In a normal Makefile build this was never an issue. But with
Ninja apparently it is...

External/pyAMI/CMakeLists.txt

@@ -38,6 +38,7 @@ function( _setup_python_package name file md5 )
 
    # Make the package target depend on this one:
    add_dependencies( Package_pyAMI ${name} )
+   add_dependencies( ${name} distribute )
 
    # Get the package directory:
    atlas_get_package_dir( pkgDir )