add CUDA build test

.gitlab-ci.yml

 before_script:
   - export LANG=C
   - module use /afs/ipp/common/usr/modules.2014/amd64_sles11/modulefiles/GPU
-  - module load cmake impi/5.1.3 cuda/7.5  
+  - module load cmake impi/5.1.3  
 
 # define two stages, that way coverage only runs after all other
 # tests have been done.
@@ -18,3 +18,14 @@ build-gcc-mpi-nocuda:
     - cd build
     - cmake .. -DCMAKE_CXX_COMPILER=g++
     - make VERBOSE=1 -j 4
+
+
+# GPU build (with CUDA)
+build-gcc-mpi-cuda:
+  stage: build
+  script:
+    - module load gcc/4.9 fftw/gcc/3.3.4 boost/gcc/1.56 cuda/7.5
+    - mkdir build
+    - cd build
+    - cmake .. -DCMAKE_CXX_COMPILER=g++ -DINCLUDE_CUDA=ON  -DCUDA_CUDA_LIBRARY=$CUDA_HOME/lib64/stubs/libcuda.so 
+    - make VERBOSE=1 -j 4