E.g. on 32-bit Linux. Issue revealed after adding i386 Linux CI build
in abdf40c #1057.

```
/home/runner/work/libssh2/libssh2/src/scp.c: In function 'scp_recv':
/home/runner/work/libssh2/libssh2/src/scp.c:765:23: error: conversion from 'libssh2_int64_t' {aka 'long long int'} to '__off_t' {aka 'long int'} may change value [-Werror=conversion]
  765 |         sb->st_size = session->scpRecv_size;
      |                       ^~~~~~~
```
Ref: https://github.com/libssh2/libssh2/actions/runs/5126803482/jobs/9221746299?pr=1054#step:12:51

Regression from 5db836b #1002
Closes #1060

E.g. on 32-bit Linux. Issue revealed after adding i386 Linux CI build
in abdf40c libssh2#1057.

```
/home/runner/work/libssh2/libssh2/src/scp.c: In function 'scp_recv':
/home/runner/work/libssh2/libssh2/src/scp.c:765:23: error: conversion from 'libssh2_int64_t' {aka 'long long int'} to '__off_t' {aka 'long int'} may change value [-Werror=conversion]
  765 |         sb->st_size = session->scpRecv_size;
      |                       ^~~~~~~
```
Ref: https://github.com/libssh2/libssh2/actions/runs/5126803482/jobs/9221746299?pr=1054#step:12:51

Regression from 5db836b libssh2#1002
Closes libssh2#1060

Unless I'm missing something, it looks like `libssh2.h` has been using
`libssh2_int64_t` unconditionally since at least 2010-04-17 when
`libssh2_scp_send64()` landed via commit
be9ee70.

This makes it redundant to detect `HAVE_LONGLONG` to fallback to a
32-bit `scpRecv_size` in `libssh2_priv.h`. Then deal with possible
combinations of this flag and `strtoll()` options, which was
error-prone.

Instead, assume in `libssh2_priv.h` that we have `libssh2_int64_t`, and
use it always.

For MSVC, this means `_MSC_VER` `1310` (from year 2003) is now
required. Based on the above, this was already so before this patch.

If there happens to be no 64-bit `strtoll()` detected, fall back to the
32-bit `strtol()` (this should never happen with MSVC, and probably
neither with any other reasonably modern toolchain.)

Also make sure to set `HAVE_STRTOI64` for older, non-CMake, MSVC builds
(e.g. `Makefile.mk` or `NMakefile` ones).

Closes libssh2#1002

E.g. on 32-bit Linux. Issue revealed after adding i386 Linux CI build
in abdf40c libssh2#1057.

```
/home/runner/work/libssh2/libssh2/src/scp.c: In function 'scp_recv':
/home/runner/work/libssh2/libssh2/src/scp.c:765:23: error: conversion from 'libssh2_int64_t' {aka 'long long int'} to '__off_t' {aka 'long int'} may change value [-Werror=conversion]
  765 |         sb->st_size = session->scpRecv_size;
      |                       ^~~~~~~
```
Ref: https://github.com/libssh2/libssh2/actions/runs/5126803482/jobs/9221746299?pr=1054#step:12:51

Regression from 5db836b libssh2#1002
Closes libssh2#1060