Interrupting configure makes the build process unhappy

[catamorphism]

If I hit Ctrl-C during configure (at least on a Mac), I get:

make: *** Deleting file config.mk'`

which means that if I subsequently do make check:

Makefile:51: config.mk: No such file or directory
[snip]
make: *** No rule to make target `/mk/tests.mk'.  Stop.

I have to manually re-run ./configure to get it to work. This seems wrong, since I didn't have to run ./configure initially -- it was triggered by doing a git update and the contents of the relevant files changing.

[graydon]

This was fixed (indirectly) by 418c6bc