config-daemon.ac: detect host AR

The problem is seen when we try to use explicit host:

    ./configure --prefix=/usr --localstatedir=/var/lib --host=x86_64-pc-linux-gnu
    make V=1

Before the change:
    ar cru libstore.a nix/libstore/libstore_a-gc.o
After the change:
    x86_64-pc-linux-gnu-ar cru libstore.a

* config-daemon.ac: use AM_PROG_AR to detect host AR

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>