build: fix C source linkage

This was actually always broken as it didn't work properly unless called
after all protocols had already been added with addProtocolPath() and
addSystemProtocol(), breaking the declarative nature of the zig build
system.

This brokenness was made much worse by a recent cleanup commit however.

build.zig

@@ -73,6 +73,10 @@ pub const ScanProtocolsStep = struct {
     system_protocols: std.ArrayList([]const u8),
     targets: std.ArrayList(scanner.Target),
 
+    /// Artifacts requiring the C interface definitions to be linked in.
+    /// TODO remove this after Zig issue #131 is implemented.
+    artifacts: std.ArrayList(*zbs.LibExeObjStep),
+
     pub fn create(builder: *zbs.Builder) *ScanProtocolsStep {
         const ally = builder.allocator;
         const self = ally.create(ScanProtocolsStep) catch oom();
@@ -83,6 +87,7 @@ pub const ScanProtocolsStep = struct {
             .protocol_paths = std.ArrayList([]const u8).init(ally),
             .system_protocols = std.ArrayList([]const u8).init(ally),
             .targets = std.ArrayList(scanner.Target).init(ally),
+            .artifacts = std.ArrayList(*zbs.LibExeObjStep).init(ally),
         };
         self.targets.append(.{ .name = "wl_display", .version = 1 }) catch oom();
         return self;
@@ -108,6 +113,12 @@ pub const ScanProtocolsStep = struct {
         self.targets.append(.{ .name = global_interface, .version = version }) catch oom();
     }
 
+    /// Add the necessary C source to the compilation unit.
+    /// Once https://github.com/ziglang/zig/issues/131 we can remove this.
+    pub fn addCSource(self: *ScanProtocolsStep, obj: *zbs.LibExeObjStep) void {
+        self.artifacts.append(obj) catch oom();
+    }
+
     fn make(step: *zbs.Step) !void {
         const self = @fieldParentPtr(ScanProtocolsStep, "step", step);
         const ally = self.builder.allocator;
@@ -137,21 +148,18 @@ pub const ScanProtocolsStep = struct {
 
         // Once https://github.com/ziglang/zig/issues/131 is implemented
         // we can stop generating/linking C code.
-        for (self.protocol_paths.items) |path| {
+        for (self.protocol_paths.items) |protocol_path| {
+            const code_path = self.getCodePath(protocol_path);
             _ = try self.builder.exec(
-                &[_][]const u8{ "wayland-scanner", "private-code", path, self.getCodePath(path) },
+                &[_][]const u8{ "wayland-scanner", "private-code", protocol_path, code_path },
             );
+            for (self.artifacts.items) |artifact| {
+                artifact.addCSourceFile(code_path, &[_][]const u8{"-std=c99"});
+            }
         }
 
         self.result.path = try fs.path.join(ally, &[_][]const u8{ out_path, "wayland.zig" });
     }
-    /// Add the necessary C source to the compilation unit.
-    /// Once https://github.com/ziglang/zig/issues/131 we can remove this.
-    pub fn addCSource(self: *ScanProtocolsStep, obj: *zbs.LibExeObjStep) void {
-        for (self.protocol_paths.items) |path| {
-            obj.addCSourceFile(self.getCodePath(path), &[_][]const u8{"-std=c99"});
-        }
-    }
 
     fn getCodePath(self: *ScanProtocolsStep, xml_in_path: []const u8) []const u8 {
         const ally = self.builder.allocator;