Drop unneeded variable HEADER_CACHE that causes clang warnings.

The variable was unused and clang warned that it was not emitted to the .o file.
display +/-NCURSES_WIDECHAR when displaying ncurses version to show whether we are linked against -lncurses or -lncursesw
Backed out changeset bf84f24e512d
Fix memory leak when multiple x-label or supersedes header fields exist in the header.

In order to keep the current behavior where the value of the last header field present is the one used, free any previous value.
Remove unused variable `matched'.
Fix regression introduced by [10cc06eff52b] where the CLEARLINE() removal was not replaced by a move() to position the cursor at the correct location to draw the status bar.

closes #3619
When "make install" fails to chgrp or chmod the mutt_dotlock binary, add the reason why we are doing this so the user knows what to look for.
avoid debug warning about trailing whitespace in a base64 encoded attachment

closes #3558
allow use of sync-mailbox when the current limit has no visible messages

closes #3538
detect pipermail-style obscured return path in mbox From lines

see #1200
change semantics of mutt_is_autoview() to return 1 if there is a matching mailcap entry and either $implicit_autoview is set or the MIME type matches the user's auto_view list.

closes #3496
closes #3426
use a separate flag for mutt_hcache_store() to signal that the gettimeofday() value should be substituted, so that a 0 uidvalidity can be used by the imap driver.

closes #3501
Merge branch 'master' into guns

* master:
  use a separate flag for mutt_hcache_store() to signal that the gettimeofday() value should be substituted, so that a 0 uidvalidity can be used by the imap driver.
  change semantics of mutt_is_autoview() to return 1 if there is a matching mailcap entry and either $implicit_autoview is set or the MIME type matches the user's auto_view list.
  detect pipermail-style obscured return path in mbox From lines
  allow use of sync-mailbox when the current limit has no visible messages
  avoid debug warning about trailing whitespace in a base64 encoded attachment
  When "make install" fails to chgrp or chmod the mutt_dotlock binary, add the reason why we are doing this so the user knows what to look for.
  Fix regression introduced by [10cc06eff52b] where the CLEARLINE() removal was not replaced by a move() to position the cursor at the correct location to draw the status bar.
  Remove unused variable `matched'.
  Fix memory leak when multiple x-label or supersedes header fields exist in the header.
  Backed out changeset bf84f24e512d
  display +/-NCURSES_WIDECHAR when displaying ncurses version to show whether we are linked against -lncurses or -lncursesw
  Drop unneeded variable HEADER_CACHE that causes clang warnings.
when falling back to viewing an attachment as text in the internal pager, perform charset conversion

this change is meant to replace [392e945dfba7]

see #3234
see #3293
backout [392e945dfba7]

closes #3293
see #3234
use pkg-config to look for libidn if available.

checking <idn/idna.h> rather than including -I/usr/include/idn for Solaris 11.

closes #3624
remove PKG_CHECK_MODULE() check and always use AC_CHECK_HEADERS()

only check for <idn/idna.h> when <idna.h> isn't found

closes #3626
do not explicitly reset the terminal attributes and move the cursor in mutt_endwin().  endwin() itself should handle this.

closes #3627
add support for $mail_check_recent to mh mailbox driver

closes #3629
Merge branch 'master' into guns

* master:
  add support for $mail_check_recent to mh mailbox driver
  do not explicitly reset the terminal attributes and move the cursor in mutt_endwin().  endwin() itself should handle this.
  remove PKG_CHECK_MODULE() check and always use AC_CHECK_HEADERS()
  use pkg-config to look for libidn if available.
  backout [392e945dfba7]
  when falling back to viewing an attachment as text in the internal pager, perform charset conversion