Implement crash backtrace to stdout for Release builds on Linux

This implements a bit better handling of fatal errors on Linux. Instead of a mere "Segmentation fault" a backtrace like
```
*** opencpn (wxWidgets 2.8.12) crashed ***, see backtrace!
0x731319 in wxCrashPrint::Report() at crashprint.cpp:83
0x510122 in MyApp::OnFatalException() at chart1.cpp:2390
0x7fcc7b3629fc in wxFatalSignalHandler at ??:0
0x7fcc75317880 in  at ??:0
0x50f977 in MyApp::OnExit() at chart1.cpp:2261
0x7fcc7b30cf8c in wxEntry(int&, wchar_t**) at ??:0
0x50b040 in main at chart1.cpp:780
Aborted
```
is produced to stdout on a crash.
The standard behavior is retained for debug builds, configured with
```
cmake -DCMAKE_BUILD_TYPE=Debug ..
```
Not as good as running in gdb, especially for plug-ins, but certainly better than nothing for the packages distributed to the general non-tech public.

What do you think?