We only want to control the default visibility for our five main
installable libraries: libglib, libgthread, libgmodule, libgobject,
libgio.  We should therefore only set -fvisibility=hidden when building
those.

Use a separate substitution variable for this purpose.

Using CFLAGS directly leads to some modules built in testcases not
exporting their symbols (and then the tests fail).  It also affects the
fam file monitoring module.

Colin had originally done it this way in his visibility patch series but
I failed to understand why so I didn't copy it.  Now I do.

Also: revert changes made to two testcases in an attempt to work around
this issue.

https://bugzilla.gnome.org/show_bug.cgi?id=691756