Better checking of available solvers

Check for required auxiliary tools and always regenerate the make rules
when the set of available tools changes.

tests/Makefile

@@ -2,6 +2,23 @@ test:
 
 .PHONY: test clean refresh help
 
+TOOL_LIST := \
+	btorsim \
+	yices \
+	aigbmc \
+	avy \
+	bitwuzla \
+	boolector \
+	btormc \
+	cvc4 \
+	mathsat \
+	pono \
+	suprove \
+	yices-smt2 \
+	yices \
+	yosys-abc \
+	z3
+
 help:
 	@cat make/help.txt
 
@@ -15,5 +32,12 @@ make/rules/test/%.mk:
 	python3 make/test_rules.py $<
 
 ifneq (help,$(MAKECMDGOALS))
+
+FIND_TOOLS := $(shell \
+	TOOLS=$$(which $(TOOL_LIST) 2>/dev/null || true); \
+	echo $$TOOLS | cmp -s make/rules/found_tools || echo $$TOOLS > make/rules/found_tools \
+)
+
 include make/rules/collect.mk
+
 endif