Windows fixes

[comex]

Mostly for msys2 and mingw64 builds, which are different from each other and different from standard Visual Studio builds. Isn't Windows fun?

• Define _GNU_SOURCE in more files (it's already used in ggml.c for Linux's sake).

• Don't use PrefetchVirtualMemory if not building for Windows 8 or later (mingw64 doesn't by default). But warn the user about this situation since it's probably not intended.

• Check for NOMINMAX already being defined, which it is on mingw64.

• Actually use the increment variable (bug in my pizza PR).

• Suppress unused variable warnings in the fake pthread_create and pthread_join implementations for Windows.

• (not Windows-related) Remove mention of asprintf from comment; asprintf is no longer used.

Fixes #871.

[prusnak]

Is there a way how to add MSYS2 and MINGW64 builds to the CI?

[anzz1]

There is a better portable way to do this:

#ifdef _WIN32
#pragma comment(lib,"kernel32.lib")
typedef struct _WIN32_MEMORY_RANGE_ENTRY {
  void*  VirtualAddress;
  size_t NumberOfBytes;
} WIN32_MEMORY_RANGE_ENTRY, *PWIN32_MEMORY_RANGE_ENTRY;
extern "C" __declspec(dllimport) void * __stdcall GetProcAddress(void * hModule, const char * lpProcName);
extern "C" __declspec(dllimport) void * __stdcall GetModuleHandleA(const char * lpModuleName);
typedef int (__stdcall *LPFN_PREFETCHVIRTUALMEMORY)(void * hProcess, size_t NumberOfEntries, PWIN32_MEMORY_RANGE_ENTRY VirtualAddresses, unsigned long Flags);
LPFN_PREFETCHVIRTUALMEMORY fnPrefetchVirtualMemory = 0;
int _fnPrefetchVirtualMemory(void * hProcess, size_t NumberOfEntries, PWIN32_MEMORY_RANGE_ENTRY VirtualAddresses, unsigned long Flags)
{
  if(!fnPrefetchVirtualMemory) {
    fnPrefetchVirtualMemory = (LPFN_PREFETCHVIRTUALMEMORY)GetProcAddress(GetModuleHandleA("kernel32"),"PrefetchVirtualMemory");
    if(!fnPrefetchVirtualMemory) {
      printf("PrefetchVirtualMemory is not supported, use --no-mmap\n");
      return 0;
    }
  }
  return fnPrefetchVirtualMemory(hProcess, NumberOfEntries, VirtualAddresses, Flags);
}
#endif

then just use _fnPrefetchVirtualMemory in place of PrefetchVirtualMemory and it'll work on all compilers and windows versions.

the cost difference is nil because getprocaddress is used anyway when walking the import address table with regular (static) imports.

edit: also a function call can be saved by using (void *)-1 in place of GetCurrentProcess()