Teach the Plugin test to check that the NO_SONAME target property works
as documented. Check that the IMPORTED targets are written with the
correct properties. When readelf is available use it to check the
actual binary files for SONAME fields.
If an executable marks symbols with __declspec(dllexport) then VS
creates an import library for it. However, it forgets to create the
directory that will contain the import library if it is different from
the location of the executable. We work around this VS bug by creating
a pre-build event on the executable target to make the directory.