Use cppdialect "C++17" instead of per-action buildoptions

Premake 5.0.0-alpha12 (released in August 2017) introduced the
`cppdialect` directive, which provides a cleaner and more portable way
to specify the C++ standard across compilers and platforms.

Previously, we used manual per-action buildoptions:

filter "action:vs*" buildoptions { "/std:c++17" }
filter "action:not vs*" buildoptions { "-std=c++17" }

This workaround was necessary due to the limitations of older Premake
versions (particularly Premake 4), which lacked built-in support for
setting the C++ language standard.

Now that we're using a newer version of Premake 5, we can simplify this
setup by using:

cppdialect "C++17"

This results in cleaner, more maintainable build scripts with consistent
behavior across toolchains.

build/premake/premake5.lua

@@ -250,13 +250,6 @@ function project_set_build_flags()
 		defines { "CONFIG2_MINIUPNPC=0" }
 	end
 
-	-- Enable C++17 standard.
-	filter "action:vs*"
-		buildoptions { "/std:c++17" }
-	filter "action:not vs*"
-		buildoptions { "-std=c++17" }
-	filter {}
-
 	-- various platform-specific build flags
 	if os.istarget("windows") then
 
@@ -437,6 +430,7 @@ function project_create(project_name, target_type)
 
 	project(project_name)
 	language "C++"
+	cppdialect "C++17"
 	kind(target_type)
 
 	filter "action:vs2017"