Enabling CUDA language optionally

CMakeLists.txt

 cmake_minimum_required(VERSION 3.8 FATAL_ERROR)
-project(openfpm_numerics LANGUAGES C CXX CUDA)
+project(openfpm_numerics LANGUAGES C CXX)
 
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/CMakeFiles/)
@@ -25,8 +25,12 @@ set(PARMETIS_DIR ${PARMETIS_ROOT})
 set(ENV{PATH} "$ENV{PATH}:${HDF5_ROOT}/bin")
 set(HDF5_PREFER_PARALLEL TRUE)
 
+if(ENABLE_GPU)
+	enable_language(CUDA)
+	find_package(CUDA)
+endif()
+
 find_package(Boost 1.52.0 REQUIRED unit_test_framework iostreams program_options)
-find_package(CUDA)
 find_package(MPI REQUIRED)
 find_package(PETSc)
 find_package(HDF5 REQUIRED)