automake: always use correct path for aclocal.real

automake: always use correct path for aclocal.real

Before this commit, it was assumed that aclocal.real is in the PATH. While
this was fine for the normal build workflow, this led to some issues if

    make TOPDIR="$(pwd)" -C "$pkgdir" compile

was called manually. The command failed with:

    /home/.../openwrt/staging_dir/host/bin/aclocal: line 2: aclocal.real: command not found
    autoreconf: /home/.../openwrt/staging_dir/host/bin/aclocal failed with exit status: 127

After the commit, the package is built sucessfully.

Signed-off-by: Leonardo Mörlein <me@irrelefant.net>