Add VERIFIC=1 make option

Signed-off-by: Clifford Wolf <clifford@clifford.at>

Makefile

-SUBDIRS = verific bigsim simple
+
+SUBDIRS := bigsim simple
+
+ifeq ($(VERIFIC),1)
+export VERIFIC=1
+SUBDIRS += verific
+else
+export YOSYS_NOVERIFIC=1
+endif
 
 all: $(addsuffix /.stamp,$(SUBDIRS))
 	echo; find * -name "*.status" | sort | xargs grep -H . | sed 's,^,    ,; s,.status:,\t,; s,PASS,pass,;' | expand -t40; echo