parser: skip $if windows etc blocks on a different os

vlib/builtin/utf8.v

@@ -85,7 +85,7 @@ pub fn (_rune string) utf32_code() int {
 	for i := 1; i < _rune.len; i++ {
 		c := int(_rune[i])
 		res = res << shift
-		res |= c & 63 /* 0x3f */
+		res |= c & 63 // 0x3f
 		shift = 6
 	}
 	return res

vlib/compiler/comptime.v

@@ -31,12 +31,15 @@ fn (p mut Parser) comp_time() {
 			}
 			p.check(.lcbr)
 			os := os_from_string(name)
-			if false && p.fileis('runtime.v') && os != p.os {
+			if (!not && os != p.os) || (not && os == p.os) {
 				// `$if os {` for a different target, skip everything inside
 				// to avoid compilation errors (like including <windows.h>
 				// on non-Windows systems)
 				mut stack := 1
 				for {
+					if p.tok == .key_return {
+						p.returns = true
+					}	
 					if p.tok == .lcbr {
 						stack++
 					} else if p.tok == .rcbr {