Updated Linux binaries to use libpng 1.6 with wxwidgets (helps #89).

The builtin library is now using separate symbols to avoid conflicts with
system libpng.
Added exit call to avoid crash on Linux when closing the IDE.

The crash appears to happen in Unlink when debugging is in progress or
after an external process is executed. In some cases it also happens in
wxluaO_deletegcobject after simply closing the app on 64bit Linux.

1 0x0000... in wxEvtHandler::Unlink()
2 0x0000... in wxEvtHandler::~wxEvtHandler()
3 0x0000... in wxProcess::~wxProcess()
4 0x0000... in wxluaO_deletegcobject(lua_State*, int, int)
5 0x0000... in wxlua_wxLuaBindClass__gc(lua_State*)