Add optional LTO and harden Windows clipboard handling

- add --with-lto support to Premake 4 and 5 release builds
  - use ThinLTO with Clang, automatic LTO with GCC, and LTCG with MSVC
  - exclude Emscripten builds from LTO configuration
  - retry SDL2 clipboard writes on Windows when the clipboard is temporarily unavailable
  - make the UIMergeView copy regression test independent of the system clipboard

premake4.lua

@@ -43,6 +43,10 @@ function newclangtoolchain(toolchain)
 	}
 end
 
+function is_clang()
+	return _OPTIONS.platform == "clang" or os.is_real("macosx") or os.is_real("ios")
+end
+
 newplatform {
 	name = "clang",
 	description = "Clang",
@@ -182,6 +186,7 @@ newoption {
     description = "Set the shared data directory (default: /usr/share/ecode)",
 }
 newoption { trigger = "with-static-cpp", description = "Builds statically libstdc++" }
+newoption { trigger = "with-lto", description = "Enables Link-Time Optimization for release builds." }
 
 function explode(div,str)
 	if (div=='') then return false end
@@ -747,6 +752,23 @@ function parse_args()
 	if _OPTIONS["time-trace"] then
 		buildoptions { "-ftime-trace" }
 	end
+
+	if _OPTIONS["with-lto"] and not os.is_real("emscripten") then
+		configuration "release"
+
+		if is_vs() then
+			buildoptions { "/GL" }
+			linkoptions { "/LTCG" }
+		elseif is_clang() then
+			buildoptions { "-flto=thin" }
+			linkoptions { "-flto=thin" }
+		else
+			buildoptions { "-flto=auto" }
+			linkoptions { "-flto=auto" }
+		end
+
+		configuration {}
+	end
 end
 
 function add_static_links()