scanner: make 0o prefix the only way to define octals

vlib/v/gen/x64/elf.v

@@ -92,7 +92,7 @@ pub fn (g mut Gen) generate_elf_footer() {
 	mut f := os.create(g.out_name) or {
 		panic(err)
 	}
-	os.chmod(g.out_name, 0775) // make it an executable
+	os.chmod(g.out_name, 0o775) // make it an executable
 	f.write_bytes(g.buf.data, g.buf.len)
 	f.close()
 	println('x64 elf binary has been successfully generated')

vlib/v/scanner/scanner.v

@@ -340,6 +340,19 @@ pub fn (s mut Scanner) scan() token.Token {
 	}
 	// `123`, `.123`
 	else if c.is_digit() || (c == `.` && nextc.is_digit()) {
+		if !s.inside_string {
+		    // In C ints with `0` prefix are octal (in V they're decimal), so discarding heading zeros is needed.
+			mut start_pos := s.pos
+			for start_pos < s.text.len && s.text[start_pos] == `0` {
+				start_pos++
+			}
+			mut prefix_zero_num := start_pos - s.pos  // how many prefix zeros should be jumped
+			// for 0b, 0o, 0x the heading zero shouldn't be jumped
+			if c == `0` && start_pos < s.text.len && !s.text[start_pos].is_digit() {
+				prefix_zero_num--
+			}
+			s.pos += prefix_zero_num  // jump these zeros
+		}
 		num := s.ident_number()
 		return s.scan_res(.number, num)
 	}