diff: mmake: if [ -d Mercury ] then enable --use-subdirs

[Fergus Henderson]

Estimated hours taken: 0.25

scripts/mmake.in:
	If the current directory contains a `Mercury' subdirectory,
	then enable `--use-subdirs' by default.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.