Makefile: fix variable declaration, comment and semicolon in value and...

Makefile: fix variable declaration, comment and semicolon in value and rule/command semicolon separator

- Add override keyword
- Add :::= and != assignment operators
- Fix variable name
A variable name may be any sequence of characters not containing ‘:’, ‘#’, ‘=’, or whitespace.
```
???=value
^^ variable
  ^^ operator
```
- Fix comment in value
```mk
var=value#comment
var=value\#nocomment
```
- Fix semicolon in value (not a special character)
- Add one-line rule
```mk
target: file1 ; cmd ...
target: file1
	cmd ...
```
- Fix quote in some contexts
```mk
$(info "'")
```

BUG: 466015