Minimum libCurl version for git

See $gmane/112765 for background.

Git uses 31 CURL_, CURLINFO and CURLOPT symbols, two of these (CURLINFO_HTTP_CODE and CURLOPT_INFILE) are officially deprecated, but only because they have been renamed in 'recent' versions.

We protect the usage of symbols introduced in version 7.9.2 and later with #ifdefs. These date back to some time in 2005 when those versions of libCurl were 3 or so years old.

We use CURLOPT_FTP_USE_EPSV unprotected, which was introduced in version 7.9.2. This is something of a pity as it is an optional configuration item which is probably not widely used (I don't think I knew there was any support for git over ftp). Still, 7.9.2 was a long time ago (Dec 2001), and no-one is complaining. Disregarding this, we would be able to use libCurl versions as far back as 7.8.1 (Aug 2001), bugs, security fixes and performance notwithstanding.

According to Daniel's list [1], CURLOPT_SSLKEY was introduced in 7.9.3, but we enable it in http.c if we see version >= 7.9.2. This could be a typo in the haxx.se list, or the option could have been available in (some) 7.9.2 releases, or it could be a git bug. Again, not one which appears to be biting anyone.

Going forward there are various options:

1. Do nothing - go with the status quo.

2. Correct the #ifdefs for CURLOPT_SSLKEY

3. Drop the #ifdefs for CURLOPT_SSLKEY entirely and make 7.9.3 our
minimum supported version. I feel slightly embarrassed about that, as
that's exactly the version I have here on AIX (unless I wrest it back
from being sysadmin-installed to being user-supported). Add a check to
the Makefile and error if libCurl is too old.

4. Drop all current #ifdefs and one of the deprecated symbol names.
Our minimum supported libCurl version would be 7.9.8 from Jun 2002.

5. Drop all current #ifdefs and both of the deprecated symbol names.
Our minimum supported libCurl version would be 7.10.8 from Nov 2003.

6. Warn (not error) if libCurl is older than say the 3 years suggested
by Daniel. This would seem to require periodic updates to the Makefile
check.

I'm happy to whip up a patch if required, but I thought a series of mutually-exclusive alternative patches would be confusing without prior agreement on the approach.

Mike

[1]  http://cool.haxx.se/cvs.cgi/curl/docs/libcurl/symbols-in-versions?rev=HEAD&content-type=text/vnd.viewcvs-markup