emacs: prefer notmuch-emacs-version in User-Agent: header

Now that we have `notmuch-emacs-version' defined in notmuch emacs MUA
use that as a part of User-Agent: header to provide more accurate
version information when sending emails.

In case some incomplete installation of notmuch emacs MUA is used and
`notmuch-emacs-version' is defined as "unknown" then fall back to ask
version info from cli (as it used to be before this commit).

Requiring notmuch-version[.elc] and if that is missing setting
"fallback" notmuch-emacs-version (to "unknown") was moved from
notmuch.el to notmuch-lib.el as notmuch-mua.el (which provides
User-Agent: information) require's the latter.

emacs/notmuch-lib.el

@@ -25,6 +25,10 @@
 (require 'mm-decode)
 (require 'cl)
 
+(unless (require 'notmuch-version nil t)
+  (defconst notmuch-emacs-version "unknown"
+    "Placeholder variable when notmuch-version.el[c] is not available."))
+
 (autoload 'notmuch-jump-search "notmuch-jump"
   "Jump to a saved search by shortcut key." t)
 

emacs/notmuch-mua.el

@@ -118,7 +118,10 @@ Note that these functions use `mail-citation-hook' if that is non-nil."
 
 (defun notmuch-mua-user-agent-notmuch ()
   "Generate a `User-Agent:' string suitable for notmuch."
-  (concat "Notmuch/" (notmuch-cli-version) " (http://notmuchmail.org)"))
+  (let ((notmuch-version (if (string= notmuch-emacs-version "unknown")
+			     (notmuch-cli-version)
+			   notmuch-emacs-version)))
+    (concat "Notmuch/" notmuch-version " (http://notmuchmail.org)")))
 
 (defun notmuch-mua-user-agent-emacs ()
   "Generate a `User-Agent:' string suitable for notmuch."

emacs/notmuch.el

@@ -61,10 +61,6 @@
 (require 'notmuch-message)
 (require 'notmuch-parser)
 
-(unless (require 'notmuch-version nil t)
-  (defconst notmuch-emacs-version "unknown"
-    "Placeholder variable when notmuch-version.el[c] is not available."))
-
 (defcustom notmuch-search-result-format
   `(("date" . "%12s ")
     ("count" . "%-7s ")