Add check and error message for cuda availability

CMakeLists.txt

 # project name and specify the minumum cmake version.
 cmake_minimum_required (VERSION 3.11)
 
+project (psrdada_cpp CXX)
 # Compiling and build options
 option(ENABLE_CUDA " Compile with .cu and .cuh files the nvcc compiler. Requires CUDA toolkit to be installed." ON)
 option(ENABLE_TESTING "Build and enable testing" ON)
@@ -8,13 +9,19 @@ option(ENABLE_BENCHMARK "Build and enable benchmark" OFF)
 option(BUILD_SUBMODULES "Build submodules" ON)
 option(BUILD_DADAFLOW "Build dadaflow" OFF)
 
-if(ENABLE_CUDA)
-    project (psrdada_cpp CXX CUDA)
+include(CheckLanguage)
+check_language(CUDA)
+
+if(ENABLE_CUDA AND NOT CMAKE_CUDA_COMPILER)
+    message(FATAL_ERROR "Cuda enabled but no cuda compiler found. Either specify CMAKE_CUDA_COMPILER manually or set ENABLE_CUDA=FALSE")
+endif()
+
+
+if(ENABLE_CUDA AND CMAKE_CUDA_COMPILER)
+    enable_language(CUDA)
     if(NOT DEFINED CMAKE_CUDA_ARCHITECTURES)
         set(CMAKE_CUDA_ARCHITECTURES "75;80;89")
     endif()
-elseif(NOT ENABLE_CUDA)
-    project (psrdada_cpp CXX)
 endif()
 
 # The version number.

[Niclas Esser]

According to this article project (psrdada_cpp CXX CUDA) should not be used when CUDA is optional.