Auto merge of #24668 - tuncer:configure-media-stack-v2, r=asajeffrey

Allow build to explicitly set the media stack

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_[This is based on @asajeffrey's work in #23423, and is a rebase and continuation of that branch]_

This allows `./mach build` to override the media stack, which is currently hard-wired based on the target. To skip gstreamer, run `./mach build -d --media-stack=dummy`.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #23050 (GitHub issue number if applicable)
<!-- Either: -->
- [x] These changes do not require tests because it's build infra

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components/servo/Cargo.toml

@@ -36,6 +36,8 @@ webgl_backtrace = [
     "canvas_traits/webgl_backtrace",
 ]
 vslatestinstalled = ["script/vslatestinstalled"]
+media-dummy = ["servo-media-dummy"]
+media-gstreamer = ["servo-media-gstreamer", "gstreamer"]
 
 [dependencies]
 background_hang_monitor = {path = "../background_hang_monitor"}
@@ -71,6 +73,8 @@ script_traits = {path = "../script_traits"}
 servo_config = {path = "../config"}
 servo_geometry = {path = "../geometry"}
 servo-media = {git = "https://github.com/servo/media"}
+servo-media-dummy = {git = "https://github.com/servo/media", optional = true}
+servo-media-gstreamer = {git = "https://github.com/servo/media", optional = true}
 servo_url = {path = "../url"}
 sparkle = "0.1"
 style = {path = "../style", features = ["servo"]}
@@ -84,16 +88,10 @@ webvr_traits = {path = "../webvr_traits"}
 webxr-api = {git = "https://github.com/servo/webxr"}
 webxr = {git = "https://github.com/servo/webxr"}
 surfman = { version = "0.1", features = ["sm-osmesa"] }
+gstreamer = { version = "0.14.5", optional = true }
 
 [target.'cfg(all(not(target_os = "windows"), not(target_os = "ios"), not(target_os="android"), not(target_arch="arm"), not(target_arch="aarch64")))'.dependencies]
 gaol = "0.2.1"
 
-[target.'cfg(any(target_os = "android", target_arch = "x86_64", target_os = "windows"))'.dependencies]
-gstreamer = "0.14.5"
-servo-media-gstreamer= {git = "https://github.com/servo/media"}
-
-[target.'cfg(not(any(target_os = "android", target_arch = "x86_64", target_os = "windows")))'.dependencies.servo-media-dummy]
-git = "https://github.com/servo/media"
-
 [target.'cfg(target_os = "windows")'.dependencies]
 mozangle = {version = "0.2"}

components/servo/lib.rs

@@ -135,7 +135,7 @@ pub use msg::constellation_msg::TopLevelBrowsingContextId as BrowserId;
 pub use servo_config as config;
 pub use servo_url as url;
 
-#[cfg(any(target_os = "android", target_arch = "x86_64", target_os = "windows",))]
+#[cfg(feature = "media-gstreamer")]
 mod media_platform {
     use super::ServoMedia;
     use servo_media_gstreamer::GStreamerBackend;
@@ -245,7 +245,7 @@ mod media_platform {
     }
 }
 
-#[cfg(not(any(target_os = "android", target_arch = "x86_64", target_os = "windows",)))]
+#[cfg(feature = "media-dummy")]
 mod media_platform {
     use super::ServoMedia;
     pub fn init() {