Don't try to create cdefghij.klm inside the app bundle on OS X when sandboxed

Actually, we check HAVE_FEATURE_READONLY_INSTALLSET and not
HAVE_FEATURE_MACOSX_SANDBOX, but in practice we set the former only
for the latter case.

We could do this test cross-platform but I didn't bother now. Nobody
uses --enable-readonly-installset when building for Linux or Windows
anyway, I think.

(There must be more places where we could and should check
HAVE_FEATURE_READONLY_INSTALLSET.)

Change-Id: Ie54f436616a1db3119b0bc0148149784127b156d