mutt: use "sendmail" instead of "/usr/bin/sendmail"

That way sending mail with mutt works on NixOS too, without having to
manually change mutt's config on your system. sendmail will be found in
$PATH at runtime, and thus the setuid wrapper will be used, if
available.

pkgs/applications/networking/mailreaders/mutt/default.nix

@@ -39,6 +39,9 @@ stdenv.mkDerivation rec {
   configureFlags = [
     "--with-mailpath=" "--enable-smtp"
 
+    # Look in $PATH at runtime, instead of hardcoding /usr/bin/sendmail
+    "ac_cv_path_SENDMAIL=sendmail"
+
     # This allows calls with "-d N", that output debug info into ~/.muttdebug*
     "--enable-debug"