opencl-clhpp: fix install directory

Previously, it was installing to `$out/include/include`. Now it's correct:

/nix/store/alw8idc8qhfhzw98banq2l5hmrdaglgm-opencl-clhpp-2.0.11
├── include
│   └── CL
│       ├── cl2.hpp
│       └── cl.hpp
└── nix-support
    └── propagated-build-inputs

3 directories, 3 files

pkgs/development/libraries/opencl-clhpp/default.nix

@@ -15,9 +15,10 @@ stdenv.mkDerivation rec {
 
   propagatedBuildInputs = [ opencl-headers ];
 
-  preConfigure = ''
-    cmakeFlags="-DCMAKE_INSTALL_PREFIX=$out/include -DBUILD_EXAMPLES=OFF -DBUILD_TESTS=OFF"
-  '';
+  cmakeFlags = [
+    "-DBUILD_EXAMPLES=OFF"
+    "-DBUILD_TESTS=OFF"
+  ];
 
   meta = with stdenv.lib; {
     description = "OpenCL Host API C++ bindings";