afuse.c: do not include alloca.h

FreeBSD doesn't like it, and glibc doesn't seem to need it.
configure.ac: bump version to 0.2.1 (+ AUTHORS)

I believe OSX support is a "big enough" milestone, and I broke
the build process (see e1d95aaa3674014483175e5c5d0ec69c6adf5dde).