notmuch: fix darwin build

notmuch tries to compile the ruby binding with custom CFLAGS which
collides with the CFLAGS created by Ruby mkmf.
This caused notmuch to fail on Darwin, because it requires `-fdeclspec`
and it was removed from CFLAGS.

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

@@ -42,6 +42,10 @@ stdenv.mkDerivation rec {
 
     substituteInPlace lib/Makefile.local \
       --replace '-install_name $(libdir)' "-install_name $out/lib"
+
+    # do not override CFLAGS of the Makefile created by mkmf
+    substituteInPlace bindings/Makefile.local \
+      --replace 'CFLAGS="$(CFLAGS) -pipe -fno-plt -fPIC"' ""
   '' + lib.optionalString withEmacs ''
     substituteInPlace emacs/notmuch-emacs-mua \
       --replace 'EMACS:-emacs' 'EMACS:-${emacs}/bin/emacs' \