makefile: Recompile V itself also for nix, if needed

* Only self rebuild if v.c and v HEAD versions differ

Makefile

@@ -14,8 +14,15 @@ all:
 ifdef WIN32
 	$(CC) -std=gnu11 -w -o v0.exe vc/v_win.c
 	./v0.exe -o v.exe compiler
+	rm -f v0.exe
 else
 	$(CC) -std=gnu11 -w -o v vc/v.c -lm
+	@(VC_V=`./v version | cut -f 3 -d " "`; \
+	V_V=`git rev-parse --short HEAD`; \
+	if [ $$VC_V != $$V_V ]; then \
+		echo "Self rebuild ($$VC_V => $$V_V)"; \
+		./v -o v compiler; \
+	fi)
 endif
 	rm -rf vc/
 	@echo "V has been successfully built"