Alternate recipes in case building blocks and/or streets mods are installed

If building blocks installed, craft:
```
G T   or...   T G
T G           G T
```
T = tar, G = gravel.  Yields 4 slates.

If streets installed, craft:
```
G A   or...   A G
A G           G A
```
A = asphalt block, G = gravel.  Yields 4 slates.

If both are installed, all of the above recipes work.

If neither mod exists, cook one gravel to get one slate, as before.