cgen: add support for `v -cmain SDL_main sdl_example.v`

cmd/v/help/build-c.txt

@@ -25,7 +25,20 @@ see also `v help build`.
 
    -cstrict
       Turn on additional C warnings. This slows down compilation
-      slightly (~10% for gcc), but sometimes provides better diagnosis.
+      slightly (~10% for gcc), but sometimes provides better error diagnosis.
+
+   -cmain <MainFunctionName>
+      Useful with framework like code, that uses macros to re-define `main`, like SDL2 does for example.
+      With that option, V will always generate:
+        `int MainFunctionName(int ___argc, char** ___argv) {` , for the program entry point function, *no matter* the OS.
+      Without it, on non Windows systems, it will generate:
+        `int main(int ___argc, char** ___argv) {`
+        ... and on Windows, it will generate:
+          a) `int WINAPI wWinMain(HINSTANCE instance, HINSTANCE prev_instance, LPWSTR cmd_line, int show_cmd){`
+          when you are compiling applications that `import gg`.
+        ... or it will generate:
+          b) `int wmain(int ___argc, wchar_t* ___argv[], wchar_t* ___envp[]){`
+          when you are compiling console apps.
 
    -showcc
       Prints the C command that is used to build the program.

vlib/v/gen/c/cmain.v

@@ -50,7 +50,11 @@ fn (mut g Gen) gen_vlines_reset() {
 	}
 }
 
-fn (mut g Gen) gen_c_main_function_header() {
+fn (mut g Gen) gen_c_main_function_only_header() {
+	if g.pref.cmain != '' {
+		g.writeln('int ${g.pref.cmain}(int ___argc, char** ___argv){')
+		return
+	}
 	if g.pref.os == .windows {
 		if g.is_gui_app() {
 			$if msvc {
@@ -65,13 +69,17 @@ fn (mut g Gen) gen_c_main_function_header() {
 			g.writeln('\tcmd_line_to_argv CommandLineToArgvW = (cmd_line_to_argv)GetProcAddress(shell32_module, "CommandLineToArgvW");')
 			g.writeln('\tint ___argc;')
 			g.writeln('\twchar_t** ___argv = CommandLineToArgvW(full_cmd_line, &___argc);')
-		} else {
+			return
+		}
 		// Console application
 		g.writeln('int wmain(int ___argc, wchar_t* ___argv[], wchar_t* ___envp[]){')
+		return
 	}
-	} else {
 	g.writeln('int main(int ___argc, char** ___argv){')
 }
+
+fn (mut g Gen) gen_c_main_function_header() {
+	g.gen_c_main_function_only_header()
 	g.writeln('\tg_main_argc = ___argc;')
 	g.writeln('\tg_main_argv = ___argv;')
 	if g.nr_closures > 0 {

vlib/v/pref/pref.v

@@ -163,6 +163,7 @@ pub mut:
 	bare_builtin_dir string // Set by -bare-builtin-dir xyz/ . The xyz/ module should contain implementations of malloc, memset, etc, that are used by the rest of V's `builtin` module. That option is only useful with -freestanding (i.e. when is_bare is true).
 	no_preludes      bool   // Prevents V from generating preludes in resulting .c files
 	custom_prelude   string // Contents of custom V prelude that will be prepended before code in resulting .c files
+	cmain            string // The name of the generated C main function. Useful with framework like code, that uses macros to re-define `main`, like SDL2 does. When set, V will always generate `int THE_NAME(int ___argc, char** ___argv){`, *no matter* the platform.
 	lookup_path      []string
 	output_cross_c   bool // true, when the user passed `-os cross`
 	output_es5       bool
@@ -686,6 +687,10 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 				res.custom_prelude = prelude
 				i++
 			}
+			'-cmain' {
+				res.cmain = cmdline.option(current_args, '-cmain', '')
+				i++
+			}
 			else {
 				if command == 'build' && is_source_file(arg) {
 					eprintln('Use `v $arg` instead.')