Add ACLOCAL_DIR setting and AM_PROG_AR macro for compiling with Homebrew
under Mac OS X.

Applies essentially the same change to
{libsigrok,libsigrokdecode,sigrok-cli}/{autogen.sh,configure.ac}

It may be that the same fix is needed for the other autogen.sh and/or
configure.ac files, but that hasn't been tested and therefore not in
this commit.