configure.ac: place build artifacts in the "build-aux" directory.

During the build, autotools generates a bunch of files that need a
place to live. By default, most of them (the "compile" and "missing"
scripts, for example) wind up in the project root. This commit sets

  AC_CONFIG_AUX_DIR([build-aux])

in configure.ac so that the aforementioned files are placed in the
build-aux directory instead. This keeps the top-level directory
nice and clean.