pkg(gtk4-layer-shell): Enable using system-installed headers for dynamic linking (#6624)

I noticed we weren't doing system-integration against the pkgconfig for
gtk4-layer-shell. This behaviour differed from how we handled system
integration for existing deps in `pkg/` (oniguruma, fontconfig).

Refactored `pkg/gtk4-layer-shell/build.zig` referencing
`pkg/oniguruma/build.zig` to use pkgconfig names in system integration.
Previously we used to libname `libgtk4-layer-shell.so`
(`gtk4-layer-shell`) instead of pkgconfig name `gtk4-layer-shell-0.pc`
which meant system integration still relied on fetching the C-headers
via `zig fetch` instead of system C-headers.

I've tested this with a `--system` build where the relevant
`.zig-cache/p/<hash of gtk4-layer-shell>` is stubbed to an empty
directory and `pkgconfig(gtk4-layer-shell-0)` is installed instead on
fedora linux.

pkg/gtk4-layer-shell/build.zig

@@ -3,23 +3,40 @@ const std = @import("std");
 // TODO: Import this from build.zig.zon when possible
 const version: std.SemanticVersion = .{ .major = 1, .minor = 1, .patch = 0 };
 
+const dynamic_link_opts: std.Build.Module.LinkSystemLibraryOptions = .{
+    .preferred_link_mode = .dynamic,
+    .search_strategy = .mode_first,
+};
+
 pub fn build(b: *std.Build) !void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const upstream = b.dependency("gtk4_layer_shell", .{});
-    const wayland_protocols = b.dependency("wayland_protocols", .{});
-
     // Zig API
     const module = b.addModule("gtk4-layer-shell", .{
         .root_source_file = b.path("src/main.zig"),
         .target = target,
         .optimize = optimize,
     });
-    module.addIncludePath(upstream.path("include"));
     // Needs the gtk.h header
     module.linkSystemLibrary("gtk4", dynamic_link_opts);
 
+    if (b.systemIntegrationOption("gtk4-layer-shell", .{})) {
+        module.linkSystemLibrary("gtk4-layer-shell-0", dynamic_link_opts);
+    } else {
+        _ = try buildLib(b, module, .{
+            .target = target,
+            .optimize = optimize,
+        });
+    }
+}
+
+fn buildLib(b: *std.Build, module: *std.Build.Module, options: anytype) !*std.Build.Step.Compile {
+    const target = options.target;
+    const optimize = options.optimize;
+
+    const upstream = b.dependency("gtk4_layer_shell", .{});
+    const wayland_protocols = b.dependency("wayland_protocols", .{});
     // Shared library
     const lib = b.addSharedLibrary(.{
         .name = "gtk4-layer-shell",
@@ -29,6 +46,7 @@ pub fn build(b: *std.Build) !void {
     lib.linkLibC();
     lib.addIncludePath(upstream.path("include"));
     lib.addIncludePath(upstream.path("src"));
+    module.addIncludePath(upstream.path("include"));
 
     // GTK
     lib.linkSystemLibrary2("gtk4", dynamic_link_opts);
@@ -76,6 +94,16 @@ pub fn build(b: *std.Build) !void {
         .{ .include_extensions = &.{".h"} },
     );
 
+    // Certain files relating to session lock were removed as we don't use them
+    const srcs: []const []const u8 = &.{
+        "gtk4-layer-shell.c",
+        "layer-surface.c",
+        "libwayland-shim.c",
+        "registry.c",
+        "stolen-from-libwayland.c",
+        "stubbed-surface.c",
+        "xdg-surface-server.c",
+    };
     lib.addCSourceFiles(.{
         .root = upstream.path("src"),
         .files = srcs,
@@ -87,20 +115,5 @@ pub fn build(b: *std.Build) !void {
     });
 
     b.installArtifact(lib);
+    return lib;
 }
-
-// Certain files relating to session lock were removed as we don't use them
-const srcs: []const []const u8 = &.{
-    "gtk4-layer-shell.c",
-    "layer-surface.c",
-    "libwayland-shim.c",
-    "registry.c",
-    "stolen-from-libwayland.c",
-    "stubbed-surface.c",
-    "xdg-surface-server.c",
-};
-
-const dynamic_link_opts: std.Build.Module.LinkSystemLibraryOptions = .{
-    .preferred_link_mode = .dynamic,
-    .search_strategy = .mode_first,
-};

src/build/SharedDeps.zig

@@ -506,7 +506,7 @@ pub fn add(
                     // IMPORTANT: gtk4-layer-shell must be linked BEFORE
                     // wayland-client, as it relies on shimming libwayland's APIs.
                     if (b.systemIntegrationOption("gtk4-layer-shell", .{})) {
-                        step.linkSystemLibrary2("gtk4-layer-shell", dynamic_link_opts);
+                        step.linkSystemLibrary2("gtk4-layer-shell-0", dynamic_link_opts);
                     } else {
                         // gtk4-layer-shell *must* be dynamically linked,
                         // so we don't add it as a static library