build-sys: enable GSettings by default

build-sys: enable GSettings by default

A new paprefs release is expected soon, and it will only support
GSettings. In order to have the default configuration work with the new
paprefs version, we need to enable GSettings by default.

If both module-gconf and module-gsettings are enabled when building
PulseAudio, both modules will be part of the default configuration. That
can cause trouble, because when the GConf database is migrated to
GSettings, the old configuration in GConf is not removed, so both
module-gconf and module-gsettings will try to load modules.

Generally it's not necessary to have both modules enabled even at build
time, so let's default to having only one of them enabled.