bug#23225: 25.1.50; url-retrieve-synchronously having trouble with some https URLs

[Tao Fang]

Update: issue resolved, refer to bug#23982:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=23982

by commit:
    1a8d31123698ccf6f165e49fcfe16631d07a7aea
    Author:     Paul Eggert <eggert@cs.ucla.edu>
    AuthorDate: Wed Aug 3 01:54:20 2016 -0700
    Commit:     Paul Eggert <eggert@cs.ucla.edu>
    CommitDate: Wed Aug 3 01:55:49 2016 -0700
    
    Parent:     64edd14 Require GnuTLS 2.12.2 or later
    Containing: master
    Follows:    emacs-25.0.92 (4312)
    
    Fix non-blocking GnuTLS with slow connection
    
    Although the problem is reported for OS X (Bug#23982), it seems to
    be possible on other POSIXish platforms.
    * src/gnutls.c (emacs_gnutls_nonblock_errno) [!WINDOWSNT]:
    New function.
    (emacs_gnutls_handshake) [!WINDOWSNT]:
    Use it as the errno function, if non-blocking.
    (Fgnutls_boot): Use GNUTLS_NONBLOCK if non-blocking.

I've tested on macOS and confirm it's been fixed.