Link against libstdc++, as required by --no-copy-dt-needed-entries.

References:
 * https://bugs.gentoo.org/show_bug.cgi?id=472362
 * https://github.com/cschwan/sage-on-gentoo/issues/230

--- a/config/Makefile.SH	
+++ b/config/Makefile.SH	
@@ -66,7 +66,7 @@
   graph=plotQt;;
 Qt4)
   PLOTCFLAGS='-D__FANCY_WIN__ `pkg-config --cflags QtGui`'
-  PLOTLIBS="`pkg-config --libs QtGui`"
+  PLOTLIBS="`pkg-config --libs QtGui` -lstdc++"
   graph=plotQt4;;
 fltk)
   PLOTCFLAGS="`fltk-config --cxxflags` $X11_INC"