rcm: Use Nix store path for RCM_LIB

The shell scripts in the rcm package use a relative path in order to
find the rcm.sh library.  That works fine when rcm is installed with
nix-env -- ~/.nix-profile/share/rcm exists as expected -- but does not
work when installed via /etc/nixos/configuration.nix.  In the
system-wide case, /run/current-system/sw/share does not contain the rcm
directory.

pkgs/tools/misc/rcm/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   name = "rcm-1.2.3";
 
   src = fetchurl {
@@ -8,9 +8,18 @@ stdenv.mkDerivation rec {
     sha256 = "0gwpclbc152jkclj3w83s2snx3dcgljwr75q1z8czl3yar7d8bsh";
   };
  
-  meta = {
+  patches = [ ./fix-rcmlib-path.patch ];
+
+  postPatch = ''
+    for f in bin/*.in; do
+      substituteInPlace $f --subst-var-by rcm $out
+    done
+  '';
+
+  meta = with stdenv.lib; {
     description = "Management Suite for Dotfiles";
     homepage = https://github.com/thoughtbot/rcm;
-    license = stdenv.lib.licenses.bsd3;
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ malyn ];
   };
-}
+}