use isfinite (C99) instead of finite

* Fix the following warning
  ~git/qucs/qucs/diagrams/curvediagram.cpp:69: warning: 'finite' is deprecated: first deprecated in OS X 10.9 [-Wdeprecated-declarations]
    if(finite(*px))

* see also:
  http://erlang.org/pipermail/erlang-patches/2013-November/004406.html