Patch for eigen3 to build for tensorflow

eigen-patch.txt

--- Eigen/src/Core/arch/CUDA/Half.h	2018-06-22 18:09:44.000000000 -0400
+++ Eigen/src/Core/arch/CUDA/Half.h	2018-07-25 01:19:55.462313100 -0400
@@ -209,7 +209,7 @@
 // conversion steps back and forth.
 
 EIGEN_STRONG_INLINE __device__ half operator + (const half& a, const half& b) {
-  return __hadd(a, b);
+  return __hadd(::__half(a), ::__half(b));
 }
 EIGEN_STRONG_INLINE __device__ half operator * (const half& a, const half& b) {
   return __hmul(a, b);
@@ -218,9 +218,7 @@
   return __hsub(a, b);
 }
 EIGEN_STRONG_INLINE __device__ half operator / (const half& a, const half& b) {
-  float num = __half2float(a);
-  float denom = __half2float(b);
-  return __float2half(num / denom);
+  return __hdiv(a, b);
 }
 EIGEN_STRONG_INLINE __device__ half operator - (const half& a) {
   return __hneg(a);

How do I add it to bazel ? tried adding this to eigen.cmake but realized that this file is generated.
Then tried to add to eigen.BUILD at cc_library.defines "eigen_PATCH_FILE=eigen-patch.txt". doesn't work either .
Will you please help and let me know what I am doing wrong ?

I am also interested in patching via bazel (building for GPU)