Changes to stable-5-0 branch

[Bastian M. <bma...@we...>]

Hi,

Today I checked in some changes to the stable-5-0 branch which make 
gnuplot sources compatible with MSYS2/Mingw-w64 on Windows, as well as 
they make wxWidgets 3.0 work.  These changes should in principle be 
compatible with other platforms, but I could only check on CentOS5 & 6.
Of course it still compiles cleanly using MSVC2012 or the old MSYS/MinGW 
combination, too.

There's another patch I would like to include, which avoids warnings 
about incompatible linkage of wxEvents on Windows.  It works on CentOS6, 
too.  Could someone please verify it does not break e.g. the Mac build?

   Bastian