catgirl: fix path to openssl utility after configuring

catgirl wants to invoke the openssl utility at runtime and tries to
obtain a path to the binary as OPENSSL_BIN. This uses pkg-config's
exec_prefix which is not where binaries are installed in nixpkgs,
sadly. There is (at least as far as I know) no more appropriate
pkg-config variable unfortunately.

pkgs/applications/networking/irc/catgirl/default.nix

@@ -9,6 +9,16 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-MEm5mrrWfNp+mBHFjGSOGvvfvBJ+Ho/K+mPUxzJDkV0=";
   };
 
+  # catgirl's configure script uses pkg-config --variable exec_prefix openssl
+  # to discover the install location of the openssl(1) utility. exec_prefix
+  # is the "out" output of libressl in our case (where the libraries are
+  # installed), so we need to fix this up.
+  postConfigure = ''
+    substituteInPlace config.mk --replace \
+      "$($PKG_CONFIG --variable exec_prefix openssl)" \
+      "${lib.getBin libressl}"
+  '';
+
   nativeBuildInputs = [ ctags pkg-config ];
   buildInputs = [ libressl ncurses ];
   strictDeps = true;