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.
Merge pull request #1 from VanessaE/master

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