minor autobuild issues

I'm building the default mercurial branch and noticed a couple of minor problems with the autotools build:

enblend-enblend.o fails because there is no dependency on signature.h, so I get this error:

  enblend.cc:69:23: fatal error: signature.h: No such file or directory

The workaround is to:

  cd src; make signature.h

Next I get:

  make[1]: *** No rule to make target `layer_selection/liblayersel.a', needed by `enblend'. Stop.

So I have to do this:

  cd src/layer_selection; make liblayersel.a

After that it seems ok.