rtags: Fix configure phase errors

Add pkgconfig as buildinput, so that the install path is correctly set
with cmake. PkgConfig is an optional dependency for rtags, but they
say it's necessary if you want to replace the prefix with
CMAKE_INSTALL_PREFIX. See:
caad9ac494/cmake/BashCompletion.cmake (L13)

Furthermore, I let the configurePhase of the rtags emacs package be a
noop.

pkgs/development/tools/rtags/default.nix

@@ -1,17 +1,12 @@
-{ stdenv, lib, fetchgit, cmake, llvmPackages, openssl, writeScript, apple_sdk, bash, emacs }:
+{ stdenv, lib, fetchgit, cmake, llvmPackages, openssl, writeScript, apple_sdk, bash, emacs, pkgconfig }:
 
 stdenv.mkDerivation rec {
   name = "rtags-${version}";
   version = "2.12";
 
-  buildInputs = [ cmake llvmPackages.llvm openssl llvmPackages.clang emacs ]
+  buildInputs = [ cmake llvmPackages.llvm openssl llvmPackages.clang emacs pkgconfig ]
     ++ lib.optionals stdenv.isDarwin [ apple_sdk.libs.xpc apple_sdk.frameworks.CoreServices ];
 
-  preConfigure = ''
-    export LIBCLANG_CXXFLAGS="-isystem ${llvmPackages.clang.cc}/include $(llvm-config --cxxflags) -fexceptions" \
-           LIBCLANG_LIBDIR="${llvmPackages.clang.cc}/lib"
-  '';
-
 
   src = fetchgit {
     rev = "refs/tags/v${version}";
@@ -25,6 +20,11 @@ stdenv.mkDerivation rec {
     '';
   };
 
+  preConfigure = ''
+    export LIBCLANG_CXXFLAGS="-isystem ${llvmPackages.clang.cc}/include $(llvm-config --cxxflags) -fexceptions" \
+           LIBCLANG_LIBDIR="${llvmPackages.clang.cc}/lib"
+  '';
+
   enableParallelBuilding = true;
 
   meta = {

pkgs/top-level/emacs-packages.nix

@@ -269,8 +269,11 @@ let
 
   rtags = melpaBuild rec {
     pname = "rtags";
-    version = "2.0"; # really, it's some arbitrary git hash
+    version = "2.12";
     src = external.rtags.src;
+
+    configurePhase = ":";
+
     propagatedUserEnvPkgs = [ external.rtags ];
     fileSpecs = [ "src/*.el" ];
     inherit (external.rtags) meta;