No longer need a specialization for bool as PBD::to_string/string_to already
has specializations for bool
Remove template specialization for float as string_to/to_string handles string
representations of infinity
The new specialisation for ConfigVariable<float>::set_from_string() needs to be exportable (it gets used somehow by ARDOUR::SessionConfiguration).
If adding LIBPBD_API causes a problem for gcc, we could change it to LIBPBD_TEMPLATE_MEMBER_API
The default for export-silence-threshold is -INFINITY, written
as "-inf" (by cfgtool) into system_config. Yet parsing the config using
a std::stringstream results in "0" (due to bugs in various libc++).
