v.build_constraint: support // vtest build: false and // vtest build: true too; add tests

vlib/v/build_constraint/constraint_test.v

@@ -15,6 +15,21 @@ fn test_eval_define() {
 	assert !benv.is_define('xyz')
 }
 
+fn test_eval_true() {
+	assert benv.eval('true')!
+}
+
+fn test_eval_false() {
+	assert !benv.eval('false')!
+}
+
+fn test_eval_comment() {
+	assert benv.eval('true // some comment')!
+	assert benv.eval(' true// another comment  ...')!
+	assert !benv.eval('false // some comment')!
+	assert !benv.eval(' false// another comment  ...')!
+}
+
 fn test_eval_platforms_and_compilers() {
 	assert benv.eval('tinyc')!
 	assert benv.eval(' tinyc')!

vlib/v/build_constraint/public.v

@@ -13,6 +13,7 @@ pub mut:
 // `defines` is a list of the user defines, for example: ['abc', 'gcboehm_opt', 'gg_record', 'show_fps']
 pub fn new_environment(facts []string, defines []string) &Environment {
 	mut b := &Environment{}
+	b.facts['true'] = true
 	for f in facts {
 		b.facts[f] = true
 	}