Gwenhywfar

Fixed compiler warnings.

BUILD: Disable some warnings, started fixing compiler warnings.

ConfigMgr: Add GWENHYWFAR_CB to virtual functions.

More printf format specifier standarization from <inttypes.h>

Minor windows debug output improvement (maybe a problem with the format specifier?)

Fix another windows printf format specifier.

In this particular case, a printf format specifier for size_t is needed,
but <inttypes.h> does not have one. We define our own, inspired from here:
https://stackoverflow.com/questions/44382862/how-to-printf-a-size-t-without-warning-in-mingw-w64-gcc-7-1

Fix printf format specifiers for windows compatibility using macros from <inttypes.h>

For explanation, see e.g.
https://en.cppreference.com/w/cpp/header/cinttypes
https://stackoverflow.com/questions/16859500/mmh-who-are-you-priu64

Minor windows warnings fixed

- missing include for _mkdir
- Removed cast from void* (8 byte on x86_64) to int in debug output
- Commented out unused variable (set but not used)

Minor windows compile warnings improved

- unused static function, unused variable ifdef'd out

For the console user interface, add a log level for progress messages, controlled by a new environment variable GWEN_CPROGRESS_LOGLEVEL.