v.pref, v.builder: support `-no-std` (skips passing `-std=c99` to the C backend)

cmd/v/help/build-c.txt

@@ -237,7 +237,14 @@ see also `v help build`.
       you use -no-rsp, V will pass the C compiler options directly to the C
       compiler, on the command line, without writing an .rsp file first.
 
-   -assert aborts
+   -no-std
+      By default, V passes -std=c99 to the C backend, but some compilers do
+      not support that, even though they may be able to compile the produced
+      code, or have other options that can be tuned to allow it.
+      Passing -no-std will remove that flag, and you can then use -cflags ''
+      to pass the other options for your specific C compiler.
+
+-assert aborts
       Call abort() after an assertion failure. Debuggers usually
       install signal handlers for SIGABRT, so your program will stop and you
       will get a backtrace. If you are running your program outside of a

vlib/v/builder/cc.v

@@ -186,7 +186,10 @@ fn (mut v Builder) setup_ccompiler_options(ccompiler string) {
 	mut debug_options := ['-g']
 	mut optimization_options := ['-O2']
 	// arguments for the C compiler
-	ccoptions.args = [v.pref.cflags, '-std=gnu99']
+	ccoptions.args = [v.pref.cflags]
+	if !v.pref.no_std {
+		ccoptions.args << '-std=c99'
+	}
 	ccoptions.wargs = [
 		'-Wall',
 		'-Wextra',

vlib/v/pref/pref.v

@@ -173,6 +173,7 @@ pub mut:
 	fatal_errors        bool        // unconditionally exit after the first error with exit(1)
 	reuse_tmpc          bool        // do not use random names for .tmp.c and .tmp.c.rsp files, and do not remove them
 	no_rsp              bool        // when true, pass C backend options directly on the CLI (do not use `.rsp` files for them, some older C compilers do not support them)
+	no_std              bool        // when true, do not pass -std=c99 to the C backend
 	use_color           ColorOutput // whether the warnings/errors should use ANSI color escapes.
 	is_parallel         bool
 	error_limit         int
@@ -478,6 +479,9 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 			'-no-rsp' {
 				res.no_rsp = true
 			}
+			'-no-std' {
+				res.no_std = true
+			}
 			'-keepc' {
 				res.reuse_tmpc = true
 			}