Check autotools/automake version for OpenOCD build

Hopefully this will cut down on the number of riscv-tools-doesn't-build
questions we get. Sample output:
Starting RISC-V Toolchain build process
OpenOCD build requires at least version 2.74 of autoconf. Aborting.

(Note OpenOCD requires autoconf 2.64, but changed to 2.74 for this
test.)