spotifyd: add build_options

Instead of manually altering the Cargo.toml file, we use the feature
flags available in cargo. There are three build options now:

- alsa: ALSA backend
- pulseaudio: Pulseaudio backend
- dbus: D-Bus functionality, both mpris and keyring

This also allows for a greater granularity when specifying dependencies.
All build options are selected as default, both to keep the package the
same as it was before (alsa and pulseaudio) and because the dependency
needed by the dbus option (dbus-libs) is required by pulseaudio already.

This commit allows builders to easily disable D-Bus functionality or
either audio backend without altering the template.

srcpkgs/spotifyd/template

@@ -1,10 +1,12 @@
 # Template file for 'spotifyd'
 pkgname=spotifyd
 version=0.2.24
-revision=1
+revision=2
 build_style=cargo
+configure_args="--no-default-features"
 hostmakedepends="pkg-config"
-makedepends="alsa-lib-devel libressl-devel pulseaudio-devel dbus-devel"
+makedepends="libressl-devel $(vopt_if alsa alsa-lib-devel)
+$(vopt_if pulseaudio pulseaudio-devel) $(vopt_if dbus dbus-devel)"
 short_desc="Spotify client running as a UNIX daemon"
 maintainer="Andrew Benson <abenson+void@gmail.com>"
 license="GPL-3.0-or-later"
@@ -12,14 +14,21 @@ homepage="https://github.com/Spotifyd/spotifyd"
 distfiles="${homepage}/archive/v${version}.tar.gz"
 checksum=d3763f4647217a8f98ee938b50e141d67a5f3d33e9378894fde2a92c9845ef80
 
+build_options="alsa pulseaudio dbus"
+build_options_default="alsa pulseaudio dbus"
+
+_features="$(vopt_if alsa 'alsa_backend,')"
+_features+="$(vopt_if pulseaudio 'pulseaudio_backend,')"
+_features+="$(vopt_if dbus 'dbus_mpris,dbus_keyring')"
+
+if [ "$_features" ]; then
+	echo $_features
+	configure_args="$configure_args --features $_features"
+fi
+
 case "$XBPS_TARGET_MACHINE" in
 	aarch64-musl) broken="https://travis-ci.org/void-linux/void-packages/jobs/636076091" ;;
 esac
 
 export CFLAGS_${RUST_BUILD//-/_}="${CFLAGS_host}"
 export CC_${RUST_BUILD//-/_}="${BUILD_CC}"
-
-pre_configure() {
-	vsed -e 's/default = \[/& "dbus", "dbus-tokio", "pulseaudio_backend", "keyring", /' \
-	 -i Cargo.toml
-}