Having RTFM more carefully I've made a bit of progress. I've added this:
export AOCL_BOARD_PACKAGE_ROOT=$ALTERAOCLSDKROOT/board/c5soc
which results in this:
$ aoc --list-boards
Board list:
c5soc
$ make
/usr/bin/ld: skipping incompatible /home/jim/altera/13.1/hld/board/c5soc/host/arm32/lib/libalteracl.so when searching for -lalteracl
/home/jim/altera/13.1/hld/board/c5soc/host/arm32/lib/libalterahalmmd.so: could not read symbols: File in wrong format
collect2: ld returned 1 exit status
make: *** Error 1
$ cd device
$ aoc hello_world.cl --board c5soc
Error: aoc: Can't find a valid license for the Altera SDK for OpenCL
I've also now read about the LM_LICENSE_FILE environment variable, but since I'm using the web edition of Quartus I don't have a license.dat. Is there any way of experimenting with this stuff on a Cyclone V without buying a Stratix V license?