Avoid crash with "Avoid a crash with weak symbols for "make test""

This fixes commit 197a6acb30
which fixed commit 95b9a477b6

Also remove useless example files

examples/ex_weak.c

@@ -1,11 +0,0 @@
-#! /usr/local/bin/tcc -run
-#include <tcclib.h>
-
-extern void weak_f (void) __attribute__ ((weak));
-
-int main ()
-{
-    if (weak_f) {
-        weak_f();
-    }
-}

examples/weak_f.c

@@ -1,6 +0,0 @@
-#include <tcclib.h>
-
-void weak_f (void)
-{
-    printf("Weak\n");
-}