SOONG_NINJA_ARGS to pass ninja args to soong_ui

Cf soong_build, allow soong_ui to support additional
ninja arguments

Test: NINJA_ARGS='-d explain' nothing
Bug: N/A
Change-Id: I42827fb817bed432251e9b0b94b15a4f6f78cb03

ui/build/soong.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"strings"
 
 	"android/soong/ui/metrics"
 	soong_metrics_proto "android/soong/ui/metrics/metrics_proto"
@@ -445,6 +446,11 @@ func runSoong(ctx Context, config Config) {
 			"-f", filepath.Join(config.SoongOutDir(), ninjaFile),
 		}
 
+		if extra, ok := config.Environment().Get("SOONG_UI_NINJA_ARGS"); ok {
+			ctx.Printf(`CAUTION: arguments in $SOONG_UI_NINJA_ARGS=%q, e.g. "-n", can make soong_build FAIL or INCORRECT`, extra)
+			ninjaArgs = append(ninjaArgs, strings.Fields(extra)...)
+		}
+
 		ninjaArgs = append(ninjaArgs, targets...)
 		cmd := Command(ctx, config, "soong "+name,
 			config.PrebuiltBuildTool("ninja"), ninjaArgs...)