Various Windows and MSVC fixes
silk: Fix incorrect ifdef in debug.c
celt: Fix broken SSE pre-processor check due to typo This broke 5 years ago in 43120f00758219a784f952754f33b9788a8d731b
Replace WIN32 with _WIN32 everywhere _WIN32 is defined on all Windows platforms by every compiler that targets Windows. We do not need WIN32 at all.
repacketizer_demo: check for read errors to fix compiler warnings Actually check for read errors instead of just storing the return value in a variable that then never gets checked. Also fixes "conversion from 'size_t' to 'int', possible loss of data" compiler warnings on Windows with MSVC caused by storing the size_t returned by fread() into an int variable.
This MR contains code-only fixes, and !13 is based on top of this MR.