v.gen.c: enable again parallel cgen for `-g`, make `-g` more robust

vlib/v/gen/c/cgen.v

@@ -527,7 +527,10 @@ pub fn (mut g Gen) gen_file() {
 
 	if g.pref.is_vlines {
 		g.vlines_path = util.vlines_escape_path(g.file.path, g.pref.ccompiler)
+		g.is_vlines_enabled = true
+		g.inside_ternary = 0
 	}
+
 	g.stmts(g.file.stmts)
 	// Transfer embedded files
 	for path in g.file.embedded_files {

vlib/v/gen/c/fn.v

@@ -111,6 +111,12 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 		return
 	}
 
+	old_is_vlines_enabled := g.is_vlines_enabled
+	g.is_vlines_enabled = true
+	defer {
+		g.is_vlines_enabled = old_is_vlines_enabled
+	}
+
 	tmp_defer_vars := g.defer_vars // must be here because of workflow
 	if !g.anon_fn {
 		g.defer_vars = []string{}

vlib/v/pref/pref.v

@@ -338,7 +338,6 @@ pub fn parse_args(known_external_commands []string, args []string) (&Preferences
 			'-g' {
 				res.is_debug = true
 				res.is_vlines = true
-				res.no_parallel = true // TODO: fix `-g` with the parallel backend (line numbers are wrong)
 				res.build_options << arg
 			}
 			'-cg' {