cgen,pref,help: support explicitly passing -subsystem windows or `-…

…subsystem console` on Windows, even for non GG apps (#22480)

vlib/v/gen/c/fn.v

@@ -2804,6 +2804,11 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 }
 
 fn (mut g Gen) is_gui_app() bool {
+	match g.pref.subsystem {
+		.windows { return true }
+		.console { return false }
+		.auto {}
+	}
 	if g.pref.os == .windows {
 		if g.force_main_console {
 			return false

vlib/v/help/build/build-c.txt

@@ -51,6 +51,13 @@ see also `v help build`.
           b) `int wmain(int ___argc, wchar_t* ___argv[], wchar_t* ___envp[]){`
           when you are compiling console apps.
 
+   -subsystem <auto|console|windows>
+      Useful to change the generated main function on Windows.
+      Ignored on other platforms. The default is `auto`.
+      When set to `console` V will generate a `wmain` main function.
+      When set to `windows` V will generate a `wWinMain` main function,
+      even when you are not compiling `gg` apps.
+
    -showcc
       Prints the C command that is used to build the program.
 
@@ -353,4 +360,4 @@ see also `v help build`.
       See https://en.wikipedia.org/wiki/Floating-point_arithmetic#%22Fast_math%22_optimization
           https://learn.microsoft.com/en-us/cpp/build/reference/fp-specify-floating-point-behavior?view=msvc-170&redirectedfrom=MSDN
           https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffast-math
-          https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ffast-math
+          https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ffast-math

vlib/v/pref/pref.v

@@ -45,6 +45,14 @@ pub enum ColorOutput {
 	never
 }
 
+// Subsystem is needed for modeling passing an explicit `/subsystem:windows` or `/subsystem:console` on Windows.
+// By default it is `auto`. It has no effect on platforms != windows.
+pub enum Subsystem {
+	auto
+	console
+	windows
+}
+
 pub enum Backend {
 	c               // The (default) C backend
 	golang          // Go backend
@@ -240,6 +248,8 @@ pub mut:
 	// use_64_int bool
 	// forwards compatibility settings:
 	relaxed_gcc14 bool = true // turn on the generated pragmas, that make gcc versions > 14 a lot less pedantic. The default is to have those pragmas in the generated C output, so that gcc-14 can be used on Arch etc.
+	//
+	subsystem Subsystem // the type of the window app, that is going to be generated; has no effect on !windows
 }
 
 pub struct LineInfo {
@@ -436,6 +446,19 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 				res.eval_argument = cmdline.option(args[i..], '-e', '')
 				i++
 			}
+			'-subsystem' {
+				subsystem := cmdline.option(args[i..], '-subsystem', '')
+				res.subsystem = Subsystem.from(subsystem) or {
+					mut valid := []string{}
+					$for x in Subsystem.values {
+						valid << x.name
+					}
+					eprintln('invalid subsystem: ${subsystem}')
+					eprintln('valid values are: ${valid}')
+					exit(1)
+				}
+				i++
+			}
 			'-gc' {
 				gc_mode := cmdline.option(args[i..], '-gc', '')
 				match gc_mode {