improve emcc input finding so that it ignores -MT x

emcc

@@ -442,6 +442,11 @@ try:
   for i in range(len(newargs)): # find input files XXX this a simple heuristic. we should really analyze based on a full understanding of gcc params,
                                 # right now we just assume that what is left contains no more |-x OPT| things
     arg = newargs[i]
+
+    if i > 0:
+      prev = newargs[i-1]
+      if prev == '-MT': continue # ignore this gcc-style argument
+
     if arg.endswith(SOURCE_SUFFIXES + BITCODE_SUFFIXES + DYNAMICLIB_SUFFIXES + ASSEMBLY_SUFFIXES) or shared.Building.is_ar(arg): # we already removed -o <target>, so all these should be inputs
       newargs[i] = ''
       if os.path.exists(arg):