cgen: generate pragmas for gcc-14, to make it more relaxed by default…

…. Support `-no-relaxed-gcc14` for turning them off. (#21680)

vlib/v/gen/c/cgen.v

@@ -792,6 +792,17 @@ pub fn (mut g Gen) init() {
 		g.cheaders.writeln(g.pref.custom_prelude)
 	} else if !g.pref.no_preludes {
 		g.cheaders.writeln('// Generated by the V compiler')
+		if g.pref.relaxed_gcc14 {
+			// See https://gcc.gnu.org/gcc-14/porting_to.html#c-code-generators:
+			g.cheaders.writeln('
+#if defined __GNUC__ && __GNUC__ >= 14
+#pragma GCC diagnostic warning "-Wimplicit-function-declaration"
+#pragma GCC diagnostic warning "-Wincompatible-pointer-types"
+#pragma GCC diagnostic warning "-Wint-conversion"
+#pragma GCC diagnostic warning "-Wreturn-mismatch"
+#endif
+')
+		}
 		if g.pref.os == .wasm32 {
 			g.cheaders.writeln('#define VWASM 1')
 			// Include <stdint.h> instead of <inttypes.h> for WASM target

vlib/v/pref/pref.v

@@ -237,6 +237,8 @@ pub mut:
 	warn_about_allocs bool // -warn-about-allocs warngs about every single allocation, e.g. 'hi $name'. Mostly for low level development where manual memory management is used.
 	// temp
 	// 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.
 }
 
 pub struct LineInfo {
@@ -609,6 +611,9 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 				res.no_preludes = true
 				res.build_options << arg
 			}
+			'-no-relaxed-gcc14' {
+				res.relaxed_gcc14 = false
+			}
 			'-prof', '-profile' {
 				res.profile_file = cmdline.option(args[i..], arg, '-')
 				res.is_prof = true