open_memstream and regex on WIN32

Nobody has claimed this yet.

By default, the http_server cannot run on Windows because of open_memstream and regex not found by the Cmake

However, thanks to [vcpkg](https://vcpkg.io/en/), one can
vcpkg install fmem
vcpkg install pcre

This was useful anyway to get a GETOPT_INCLUDE_DIR/GETOPT_LIB_DIR
vcpkg install getopt

But there are a few problems :

The Cmake won't handle `CHECK_FUNCTION_EXISTS(open_memstream HAVE_OPEN_MEMSTREAM)` correctly, because :
it does not allow any OPENMEMSTREAM_INCLUDE_DIR
the vcpkg fmem does not declare `open_memstream()` function, there is a fmem abstraction

But it can be fixed easily
```
FILE* open_memstream(char** dstBuf, size_t* dstSize) {
FILE* result = NULL;
fmem stream_info;
fmem_init(&stream_info);
result = fmem_open(&stream_info, "w+");
fmem_mem(&stream_info, dstBuf, dstSize);
return result;
}
```

Then I had to force
```
#define HAVE_OPEN_MEMSTREAM 1
#define HAVE_REGEX 1
```

in test_config.h

Inspect the CMake logic around CHECK_FUNCTION_EXISTS(open_memstream...) and test_config.h, then trace how the http_server uses these feature definitions on WIN32. Reproduce the vcpkg setup for fmem, pcre, and getopt; done means the http_server builds and runs on Windows without manually forcing HAVE_OPEN_MEMSTREAM or HAVE_REGEX.

Written by the indexing model from the issue text.