Some platforms use different extensions for compile-time linkable
libraries vs runtime-loadable modules. Need to use special libtool
flag in the latter case for consistency with what gmodule expects.
https://bugzilla.gnome.org/show_bug.cgi?id=731703
If installed tests are not enabled, installed_testdir is not
defined, so we end up trying to create /modules and to chmod
things in /x-content/, which is not right.