tool/gocross: add ts_macext build tag for Xcode builds

It's used to control various opt-in functionality for the macOS and iOS
apps, and was lost in the migration to gocross.

Updates tailscale/tailscale#7769

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

tool/gocross/autoflags.go

@@ -102,6 +102,10 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		cgo = nativeGOOS == "darwin"
 		tags = append(tags, "omitidna", "omitpemdecrypt")
 		if env.IsSet("XCODE_VERSION_ACTUAL") {
+			// If we're building via Xcode, we must be making the extension
+			// version (as opposed to tailscaled on Mac).
+			tags = append(tags, "ts_macext")
+
 			var xcodeFlags []string
 			// Minimum OS version being targeted, results in
 			// e.g. -mmacosx-version-min=11.3, -miphoneos-version-min=15.0

tool/gocross/autoflags_test.go

@@ -301,7 +301,7 @@ TS_LINK_FAIL_REFLECT=0 (was <nil>)`,
 			wantArgv: []string{
 				"gocross", "build",
 				"-trimpath",
-				"-tags=tailscale_go,omitidna,omitpemdecrypt",
+				"-tags=tailscale_go,omitidna,omitpemdecrypt,ts_macext",
 				"-ldflags", "-X tailscale.com/version.longStamp=1.2.3-long -X tailscale.com/version.shortStamp=1.2.3 -X tailscale.com/version.gitCommitStamp=abcd -X tailscale.com/version.extraGitCommitStamp=defg -w",
 				"./cmd/tailcontrol",
 			},
@@ -333,7 +333,7 @@ TS_LINK_FAIL_REFLECT=1 (was <nil>)`,
 			wantArgv: []string{
 				"gocross", "build",
 				"-trimpath",
-				"-tags=tailscale_go,omitidna,omitpemdecrypt",
+				"-tags=tailscale_go,omitidna,omitpemdecrypt,ts_macext",
 				"-ldflags", "-X tailscale.com/version.longStamp=1.2.3-long -X tailscale.com/version.shortStamp=1.2.3 -X tailscale.com/version.gitCommitStamp=abcd -X tailscale.com/version.extraGitCommitStamp=defg -w",
 				"./cmd/tailcontrol",
 			},