gnu: notmuch: Add sfsexp input.

sfsexp is a C/C++ library for the manipulation (read, parse, modify, and
create) of symbolic expressions.  In an environment where sfsexp is available,
Notmuch will compile with support for a S-expression-based query format.  This
commit is to add sfsexp as a Notmuch dependency and therefore to add S-exp
support.

* gnu/packages/mail.scm (notmuch)[native-inputs]: Add git-minimal/pinned.
[inputs]: Add sfsexp.

Change-Id: Ic3b4a30b2672ad690d66b283e9bdc8e2197f97a7
Signed-off-by: Christopher Baines <mail@cbaines.net>

gnu/packages/mail.scm

@@ -82,6 +82,7 @@
   #:use-module (gnu packages bash)
   #:use-module (gnu packages bison)
   #:use-module (gnu packages boost)
+  #:use-module (gnu packages c)
   #:use-module (gnu packages calendar)
   #:use-module (gnu packages check)
   #:use-module (gnu packages compression)
@@ -1466,11 +1467,12 @@ invoking @command{notifymuch} from the post-new hook.")
            emacs-no-x           ; -minimal lacks libxml, needed for some tests
            which
            dtach
+           git-minimal/pinned
            gnupg
            man-db
            perl))
     (inputs
-     (list glib gmime talloc xapian zlib))
+     (list glib gmime sfsexp talloc xapian zlib))
     (home-page "https://notmuchmail.org/")
     (synopsis "Thread-based email index, search, and tagging")
     (description