Avoid doing CudaFreeEnergy tests when no cuda GPU is available.

Export CudaFreeEnergyKernelFactory symbols to avoid link error in test program.

plugins/freeEnergy/platforms/cuda/CMakeLists.txt

@@ -37,7 +37,10 @@ ENDIF(LOG)
 
 # ----------------------------------------------------------------------------
 
-SUBDIRS (tests)
+# Only run tests when cuda-enabled GPU is found
+IF( (NOT DEFINED CUDA_HAVE_GPU) OR (CUDA_HAVE_GPU) )
+    SUBDIRS (tests)
+ENDIF( (NOT DEFINED CUDA_HAVE_GPU) OR (CUDA_HAVE_GPU) )
 
 # The source is organized into subdirectories, but we handle them all from
 # this CMakeLists file rather than letting CMake visit them as SUBDIRS.

plugins/freeEnergy/platforms/cuda/include/CudaFreeEnergyKernelFactory.h

@@ -35,7 +35,7 @@ namespace OpenMM {
  * This KernelFactory creates all kernels for CudaFreeEnergyPlatform.
  */
 
-class CudaFreeEnergyKernelFactory : public KernelFactory {
+class OPENMM_EXPORT CudaFreeEnergyKernelFactory : public KernelFactory {
 public:
     KernelImpl* createKernelImpl(std::string name, const Platform& platform, ContextImpl& context) const;
 };