Use new premake lto api if available

Premake v5.0.0-beta4 replaces the LinkTimeOptimization flag with a
function linktimeoptimization, use it if available to avoid deprecation
warning.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

build/premake/premake5.lua

@@ -209,7 +209,12 @@ function project_set_build_flags()
 			optimize "Speed"
 		end
 		if _OPTIONS["with-lto"] then
-			flags { "LinkTimeOptimization" }
+			if linktimeoptimization then
+				linktimeoptimization("On")
+			else
+				-- deprecated since v5.0.0-beta4
+				flags { "LinkTimeOptimization" }
+			end
 		end
 		defines { "NDEBUG", "CONFIG_FINAL=1" }