The required low-level logic was already available as part of
`libssh2` (via `LIBSSH2_FLAG_COMPRESS` `libssh2_session_flag()`[1]
option.)

This patch adds the new `libcurl` option `CURLOPT_SSH_COMPRESSION`
(boolean) and the new `curl` command-line option `--compressed-ssh`
to request this `libssh2` feature. To have compression enabled, it
is required that the SSH server supports a (zlib) compatible
compression method and that `libssh2` was built with `zlib` support
enabled.

[1] https://www.libssh2.org/libssh2_session_flag.html

Ref: https://github.com/curl/curl/issues/1732
Closes https://github.com/curl/curl/pull/1735

Closes https://github.com/curl/curl/pull/1793

This change does two things:
1. It un-breaks the build in Xcode 9.0. (Xcode 9.0 is currently
   failing trying to compile connectx() in lib/connect.c.)
2. It finally weak-links the connectx() function, and falls back on
   connect() when run on older operating systems.

Closes #1790

closes #1747

... when setting clang version to assume 3.7

Caused a lot of "integer expression expected" warnings by configure.

Closes #1786

Closes https://github.com/curl/curl/issues/1784

Update the progress timers `t_nslookup`, `t_connect`, `t_appconnect`,
`t_pretransfer`, and `t_starttransfer` to track the total times for
these activities when a redirect is followed. Previously, only the times
for the most recent request would be tracked.

Related changes:

  - Rename `Curl_pgrsResetTimesSizes` to `Curl_pgrsResetTransferSizes`
    now that the function only resets transfer sizes and no longer
    modifies any of the progress timers.

  - Add a bool to the `Progress` struct that is used to prevent
    double-counting `t_starttransfer` times.

Added test case 1399.

Fixes #522 and Known Bug 1.8
Closes #1602
Reported-by: joshhe on github

Closes #1785

Commit curl-7_54_0-118-g8b2f22e changed the output format of curl --help
to use <file> and <dir> instead of FILE and DIR, which caused zsh.pl to
produce a broken completion script:

% curl --<TAB>
_curl:10: no such file or directory: seconds

Closes #1779

... make sure we bail out on overflows.

Reported-by: Brian Carpenter
Closes #1758

to make sure they keep building warning-free

Closes #1777

... converted from the MS specific _snscanf()

Closes #1719

Fixes #1764
Closes #1773
Reported-by: James Slaughter

Closes #1774

Closes #1772

This fixes redirects to IDN URLs

Fixes #1441
Closes #1762
Reported by: David Lord

Could've prevented #1755

Closes #1763

Regression since adef394a (released in 7.55.0)

Reported-by: Han Qiao
Fixes #1769
Closes #1771

Fixes #1752

Closes #1770

The long list of architectures in include/curl/system.h is annoying to
maintain, and needs to be extended for each and every architecture to
support.

Instead, let's rely on the __SIZEOF_LONG__ define of the gcc compiler
(we are in the GNUC condition anyway), which tells us if long is 4
bytes or 8 bytes.

This fixes the build of libcurl 7.55.0 on architectures such as
OpenRISC or ARC.

Closes #1766

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>