libressl: fix pkg-config exec_prefix

pkgs/development/libraries/libressl/default.nix

@@ -40,6 +40,9 @@ let
     # removing ./configure pre-config.
     preConfigure = ''
       rm configure
+      substituteInPlace CMakeLists.txt \
+        --replace 'exec_prefix \''${prefix}' "exec_prefix ${placeholder "bin"}" \
+        --replace 'libdir      \''${exec_prefix}' 'libdir \''${prefix}'
     '';
 
     inherit patches;