Require Sail version 0.18

Use the new `--require-version` flag to set the minimum version to 0.18 (the current latest release). If you have an older version that supports this flag you'll get an error like

    Sail version 0.17.1 is older than requested version 0.18

Currently there actually aren't any older versions that support this flag (unless you compiled from source) so you'd get this error instead:

    sail: unknown option '--require-version'.

Not ideal, but the problem will solve itself eventually so I don't think it's worth worrying about for now.

Makefile

@@ -129,6 +129,7 @@ SAIL_COQ_SRCS  = $(addprefix model/,$(SAIL_ARCH_SRCS) $(SAIL_SEQ_INST_SRCS) $(SA
 
 PLATFORM_OCAML_SRCS = $(addprefix ocaml_emulator/,platform.ml platform_impl.ml softfloat.ml riscv_ocaml_sim.ml)
 
+SAIL_FLAGS += --requireh-version 0.18
 SAIL_FLAGS += -dno_cast
 SAIL_DOC_FLAGS ?= -doc_embed plain