cppe: fix build with clang

pkgs/development/libraries/science/chemistry/cppe/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, fetchFromGitHub, cmake }:
+{ stdenv, lib, fetchFromGitHub, cmake, llvmPackages }:
 
 stdenv.mkDerivation rec {
   pname = "cppe";
@@ -11,7 +11,8 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-guM7+ZWDJLcAUJtPkKLvC4LYSA2eBvER7cgwPZ7FxHw=";
   };
 
-  nativeBuildInputs = [ cmake ];
+  nativeBuildInputs = [ cmake ]
+    ++ lib.optionals stdenv.cc.isClang [ llvmPackages.openmp ];
 
   cmakeFlags = [ "-DCMAKE_INSTALL_LIBDIR=lib" ];