scons: Don't require -liberty on MinGW.

Not always present.
diff --git a/scons/crossmingw.py b/scons/crossmingw.py
index 3aed484..9cb0b89 100644
--- a/scons/crossmingw.py
+++ b/scons/crossmingw.py
@@ -177,7 +177,7 @@
     # default in recent gcc versions
     env.AppendUnique(CFLAGS = ['-gstabs'])
 
-    env.AppendUnique(LIBS = ['iberty'])
+    #env.AppendUnique(LIBS = ['iberty'])
     env.AppendUnique(SHLINKFLAGS = ['-Wl,--enable-stdcall-fixup'])
     #env.AppendUnique(SHLINKFLAGS = ['-Wl,--kill-at'])