-isystem is not handled correctly

[rofl0r]

if -isystem /my/default/include is used, /my/default/include is passed unfiltered to ar invocation.

PoC patch fixing it:

--- a/mlibtool.c
+++ b/mlibtool.c
@@ -1075,6 +1075,11 @@ static void ltlink(struct Options *opt)
                 rpath = narg;
                 i++;

+            } else if (!strcmp(arg, "-isystem") && narg) {
+               if(buildLib) ; /* ignore */
+               else ;
+                i++;
+
             } else if (!strcmp(arg, "-version-info") && narg) {
                 /* current:revision:age instead of major.minor.revision */
                 int current = 0;