builder: pass `-std=gnu99` to the C backend under linux (#12236)

cmd/v/help/build-c.txt

@@ -243,7 +243,7 @@ see also `v help build`.
       compiler, on the command line, without writing an .rsp file first.
 
    -no-std
-      By default, V passes -std=c99 to the C backend, but some compilers do
+      By default, V passes -std=gnu99(linux)/-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 ''

vlib/v/builder/cc.v

@@ -162,8 +162,12 @@ fn (mut v Builder) setup_ccompiler_options(ccompiler string) {
 	// arguments for the C compiler
 	ccoptions.args = [v.pref.cflags]
 	if !v.pref.no_std {
+		if v.pref.os == .linux {
+			ccoptions.args << '-std=gnu99 -D_DEFAULT_SOURCE'
+		} else {
 			ccoptions.args << '-std=c99 -D_DEFAULT_SOURCE'
 		}
+	}
 	ccoptions.wargs = [
 		'-Wall',
 		'-Wextra',

vlib/v/pref/pref.v

@@ -176,7 +176,7 @@ 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
+	no_std              bool        // when true, do not pass -std=gnu99(linux)/-std=c99 to the C backend
 	use_color           ColorOutput // whether the warnings/errors should use ANSI color escapes.
 	no_parallel         bool
 	is_vweb             bool // skip _ var warning in templates