GN: build Lua tools when skia_use_lua.

When we opt into Lua, this builds SampleLua into SampleApp, and the lua_app and lua_pictures tools.

I've tested this builds with and without skia_use_system_lua on my Mac laptop and Linux desktop.

I've made lua_pictures.cpp's flags static to avoid conflicts with flags in SkCommonFlags.cpp.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4699

DOCS_PREVIEW= https://skia.org/?cl=4699

Change-Id: I8176fd51d8a38746e7d730cfcce66da42b9a015a
Reviewed-on: https://skia-review.googlesource.com/4699
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/tools/lua/lua_pictures.cpp b/tools/lua/lua_pictures.cpp
index bd8fda1..68a610e 100644
--- a/tools/lua/lua_pictures.cpp
+++ b/tools/lua/lua_pictures.cpp
@@ -31,13 +31,13 @@
 
 // Example usage for the modulo flag:
 // for i in {0..5}; do lua_pictures --skpPath SKP_PATH -l YOUR_SCRIPT --modulo $i 6 &; done
-DEFINE_string(modulo, "", "[--modulo <remainder> <divisor>]: only run tests for which "
+static DEFINE_string(modulo, "", "[--modulo <remainder> <divisor>]: only run tests for which "
               "testIndex %% divisor == remainder.");
-DEFINE_string2(skpPath, r, "", "Read this .skp file or .skp files from this dir");
-DEFINE_string2(luaFile, l, "", "File containing lua script to run");
-DEFINE_string2(headCode, s, "", "Optional lua code to call at beginning");
-DEFINE_string2(tailFunc, s, "", "Optional lua function to call at end");
-DEFINE_bool2(quiet, q, false, "Silence all non-error related output");
+static DEFINE_string2(skpPath, r, "", "Read this .skp file or .skp files from this dir");
+static DEFINE_string2(luaFile, l, "", "File containing lua script to run");
+static DEFINE_string2(headCode, s, "", "Optional lua code to call at beginning");
+static DEFINE_string2(tailFunc, s, "", "Optional lua function to call at end");
+static DEFINE_bool2(quiet, q, false, "Silence all non-error related output");
 
 static sk_sp<SkPicture> load_picture(const char path[]) {
     std::unique_ptr<SkStream> stream = SkStream::MakeFromFile(path);