neomutt: fix sendmail default value

NeoMutt doesn't use autoconf any more, so the autoconf variable in
configureFlags wasn't doing anything.  With the new autosetup build
system, there doesn't seem to be a way to override the default
sendmail location, so we have to patch it.

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

@@ -27,6 +27,7 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   postPatch = ''
+    substituteInPlace auto.def --replace /usr/sbin/sendmail sendmail
     substituteInPlace contrib/smime_keys \
       --replace /usr/bin/openssl ${openssl}/bin/openssl
 
@@ -60,8 +61,6 @@ stdenv.mkDerivation rec {
     # To make it not reference .dev outputs. See:
     # https://github.com/neomutt/neomutt/pull/2367
     "--disable-include-path-in-cflags"
-    # Look in $PATH at runtime, instead of hardcoding /usr/bin/sendmail
-    "ac_cv_path_SENDMAIL=sendmail"
     "--zlib"
   ];