compiler: support for -color/-nocolor option overrides

cmd/v/help/build.txt

@@ -103,6 +103,12 @@ The build flags are shared by the build and run commands:
       NB: an useful, although not entirely accurate regexp based Universal Ctags options file
       for V is located in `.ctags.d/v.ctags` . If you use https://ctags.io/ , it will be used
       up automatically, or you can specify it explicitly with --options=.ctags.d/v.ctags .
+      
+   -color, -nocolor
+      Force the use of ANSI colors for the error/warning messages, or disable them completely.
+      By default V tries to show its errors/warnings in ANSI color. The heuristic that it uses
+      to detect whether or not to use ANSI colors may not work in all cases. 
+      These options allow you to override the default detection.
 
 For C-specific build flags, use `v help build-c`.
 

vlib/v/builder/builder.v

@@ -34,6 +34,12 @@ pub fn new_builder(pref &pref.Preferences) Builder {
 	rdir := os.real_path(pref.path)
 	compiled_dir := if os.is_dir(rdir) { rdir } else { os.dir(rdir) }
 	table := table.new_table()
+	if pref.use_color == .always {
+		util.emanager.set_support_color(true)
+	}
+	if pref.use_color == .never {
+		util.emanager.set_support_color(false)
+	}
 	return Builder{
 		pref: pref
 		table: table

vlib/v/pref/pref.v

@@ -20,6 +20,12 @@ pub enum OutputMode {
 	silent
 }
 
+pub enum ColorOutput {
+	auto
+	always
+	never
+}    
+
 pub enum Backend {
 	c            // The (default) C backend
 	js           // The JavaScript backend
@@ -101,6 +107,7 @@ pub mut:
 	print_v_files       bool     // when true, just print the list of all parsed .v files then stop.
 	skip_running        bool     // when true, do no try to run the produced file (set by b.cc(), when -o x.c or -o x.js)
 	skip_warnings       bool     // like C's "-w"
+	use_color           ColorOutput // whether the warnings/errors should use ANSI color escapes.
 }
 
 pub fn parse_args(args []string) (&Preferences, string) {
@@ -166,6 +173,12 @@ pub fn parse_args(args []string) (&Preferences, string) {
 			'-translated' {
 				res.translated = true
 			}
+			'-color' {
+				res.use_color=.always
+			}
+			'-nocolor' {
+				res.use_color=.never
+			}
 			'-showcc' {
 				res.show_cc = true
 			}

vlib/v/util/errors.v

@@ -30,7 +30,7 @@ pub const (
 )
 
 pub struct EManager {
-pub mut:
+mut:
 	support_color bool
 }
 
@@ -40,6 +40,10 @@ pub fn new_error_manager() &EManager {
 	}
 }
 
+pub fn (e &EManager) set_support_color(b bool) {
+	e.support_color = b
+}
+
 fn bold(msg string) string {
 	if !emanager.support_color {
 		return msg