build: replace lib-vt step with -Demit-lib-vt option

Remove the dedicated `zig build lib-vt` step and replace it with a
`-Demit-lib-vt` build option. This fixes two problems:

1. We can default XCFramework, app, etc. steps to false if emit-lib-vt
   is true, so that the lib-vt build doesn't pull in unrelated
   artifacts. **Most importantly, lib-vt alone can be build without
   full Xcode installations.**

2. We can build lib-vt as part of a bundle with other artifacts if we
   really want.

src/build/Config.zig

@@ -51,6 +51,7 @@ emit_bench: bool = false,
 emit_docs: bool = false,
 emit_exe: bool = false,
 emit_helpgen: bool = false,
+emit_lib_vt: bool = false,
 emit_macos_app: bool = false,
 emit_terminfo: bool = false,
 emit_termcap: bool = false,
@@ -314,11 +315,17 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
     //---------------------------------------------------------------
     // Artifacts to Emit
 
+    config.emit_lib_vt = b.option(
+        bool,
+        "emit-lib-vt",
+        "Set defaults for a libghostty-vt-only build (disables xcframework, macOS app, and docs).",
+    ) orelse false;
+
     config.emit_exe = b.option(
         bool,
         "emit-exe",
         "Build and install main executables with 'build'",
-    ) orelse true;
+    ) orelse !config.emit_lib_vt;
 
     config.emit_test_exe = b.option(
         bool,
@@ -352,7 +359,8 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
         // If we are emitting any other artifacts then we default to false.
         if (config.emit_bench or
             config.emit_test_exe or
-            config.emit_helpgen) break :emit_docs false;
+            config.emit_helpgen or
+            config.emit_lib_vt) break :emit_docs false;
 
         // We always emit docs in system package mode.
         if (system_package) break :emit_docs true;
@@ -401,7 +409,8 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
         bool,
         "emit-xcframework",
         "Build and install the xcframework for the macOS library.",
-    ) orelse builtin.target.os.tag.isDarwin() and
+    ) orelse !config.emit_lib_vt and
+        builtin.target.os.tag.isDarwin() and
         target.result.os.tag == .macos and
         config.app_runtime == .none and
         (!config.emit_bench and
@@ -412,7 +421,7 @@ pub fn init(b: *std.Build, appVersion: []const u8) !Config {
         bool,
         "emit-macos-app",
         "Build and install the macOS app bundle.",
-    ) orelse config.emit_xcframework;
+    ) orelse !config.emit_lib_vt and config.emit_xcframework;
 
     //---------------------------------------------------------------
     // System Packages

src/build/GhosttyExe.zig

@@ -29,8 +29,9 @@ pub fn init(b: *std.Build, cfg: *const Config, deps: *const SharedDeps) !Ghostty
     // Set PIE if requested
     if (cfg.pie) exe.pie = true;
 
-    // Add the shared dependencies
-    _ = try deps.add(exe);
+    // Add the shared dependencies. When building only lib-vt we skip
+    // heavy deps so cross-compilation doesn't pull in GTK, etc.
+    if (!cfg.emit_lib_vt) _ = try deps.add(exe);
 
     // Check for possible issues
     try checkNixShell(exe, cfg);

src/build/SharedDeps.zig

@@ -121,7 +121,7 @@ pub fn add(
     // We don't support cross-compiling to Darwin but due to the way
     // lazy dependencies work with Zig, we call this function. So we just
     // bail. The build will fail but the build would've failed anyways.
-    // And this lets other non-platform-specific targets like `lib-vt`
+    // And this lets other non-platform-specific targets like `-Demit-lib-vt`
     // cross-compile properly.
     if (!builtin.target.os.tag.isDarwin() and
         self.config.target.result.os.tag.isDarwin())