Fix MinGW build.

Thanks to Victor Bombi.

src/Makefile

@@ -194,7 +194,7 @@ CCOPTIONS= $(CCDEBUG) $(CCOPT) $(CCWARN) $(XCFLAGS) $(CFLAGS)
 LDOPTIONS= $(CCDEBUG) $(LDFLAGS)
 
 HOST_CC= $(CC)
-HOST_RM= rm -f
+HOST_RM?= rm -f
 # If left blank, minilua is built and used. You can supply an installed
 # copy of (plain) Lua 5.1 or 5.2, plus Lua BitOp. E.g. with: HOST_LUA=lua
 HOST_LUA=