Linux build on Linux fails

Further to my previous post with the build failing when trying to build it on Windows, I am now getting a different error building on Linux.

Exception: C++ code builder is unable to build C++ code for Linux: Could not find valid clang executable at clang
UnityEditorInternal.Runner.RunProgram (UnityEditor.Utils.Program p, System.String exe, System.String args, System.String workingDirectory, UnityEditor.Scripting.Compilers.CompilerOutputParserBase parser) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/BuildUtils.cs:129)
UnityEditorInternal.Runner.RunManagedProgram (System.String exe, System.String args, System.String workingDirectory, UnityEditor.Scripting.Compilers.CompilerOutputParserBase parser, System.Action`1[T] setupStartInfo) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/BuildUtils.cs:65)
UnityEditorInternal.IL2CPPBuilder.RunIl2CppWithArguments (System.Collections.Generic.List`1[T] arguments, System.Action`1[T] setupStartInfo) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:804)
UnityEditorInternal.IL2CPPBuilder.ConvertPlayerDlltoCpp (UnityEditor.Il2Cpp.Il2CppBuildPipelineData data) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:773)
UnityEditorInternal.IL2CPPBuilder.Run () (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:599)
UnityEditorInternal.IL2CPPUtils.RunIl2Cpp (System.String stagingAreaData, UnityEditorInternal.IIl2CppPlatformProvider platformProvider, System.Action`1[T] modifyOutputBeforeCompile, UnityEditor.RuntimeClassRegistry runtimeClassRegistry) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/IL2CPPUtils.cs:276)
DesktopStandalonePostProcessor.RunIL2CPP (UnityEditor.Modules.BuildPostProcessArgs args, UnityEditorInternal.IIl2CppPlatformProvider il2cppPlatformProvider, System.Collections.Generic.List`1[T] cppPlugins) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/DesktopStandalonePostProcessor.cs:256)
DesktopStandalonePostProcessor.SetupStagingArea (UnityEditor.Modules.BuildPostProcessArgs args, System.Collections.Generic.HashSet`1[T] filesToNotOverwrite) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/DesktopStandalonePostProcessor.cs:232)
DesktopStandalonePostProcessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/DesktopStandalonePostProcessor.cs:42)
Rethrow as BuildFailedException: Exception of type 'UnityEditor.Build.BuildFailedException' was thrown.
DesktopStandalonePostProcessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/DesktopStandalonePostProcessor.cs:60)
UnityEditor.Modules.DefaultBuildPostprocessor.PostProcess (UnityEditor.Modules.BuildPostProcessArgs args, UnityEditor.BuildProperties& outProperties) (at /home/bokken/buildslave/unity/build/Editor/Mono/Modules/DefaultBuildPostprocessor.cs:29)
UnityEditor.PostprocessBuildPlayer.Postprocess (UnityEditor.BuildTargetGroup targetGroup, UnityEditor.BuildTarget target, System.String installPath, System.String companyName, System.String productName, System.Int32 width, System.Int32 height, UnityEditor.BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport report) (at /home/bokken/buildslave/unity/build/Editor/Mono/BuildPipeline/PostprocessBuildPlayer.cs:337)
UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr, Boolean&) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:189)

Any help appreciated.

John