Attempt to enable compute 5.2 for NVIDIA GPU support

Issue #292.

cmake/checkcuda.cu

@@ -5,7 +5,7 @@
 
 int main(int argc, char **argv){
     cudaDeviceProp dP;
-    float min_cc = 5.3;  // We need half floats...
+    float min_cc = 5.2;
 
     int rc = cudaGetDeviceProperties(&dP, 0);
     if(rc != cudaSuccess) {

src/pbrt/util/float.h

@@ -499,7 +499,7 @@ class Half {
 
     PBRT_CPU_GPU
     bool operator==(const Half &v) const {
-#ifdef PBRT_IS_GPU_CODE
+#if defined(PBRT_IS_GPU_CODE) && __CUDA_ARCH__ >= 530
         return __ushort_as_half(h) == __ushort_as_half(v.h);
 #else
         if (Bits() == v.Bits())