rc/tools/make.kak: quote "${kak_opt_makecmd}"

As per man page eval(1p):
> The eval utility shall construct a command by concatenating arguments together,
> separating each with a `<space>` character.  The constructed command shall be
> read and executed by the shell.

When not quoting `$kak_opt_makecmd` in the eval, the variable is split by
newlines and spaces and then joined by spaces to form the command. If there
were newlines in `$kak_opt_makecmd`, the command would be malformed.


To reproduce:
```kak
set-option global makecmd "
echo foo
echo bar"
make a b c
```

Expected output in the `*make*` buffer:
```
foo
bar a b c
```

Actual output:
```
foo echo bar a b c
```

This patch fixes this.

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