Disable BUILD_TESTS for warpctc (#8210)

* It will sightly faster compile and make warpctc compile well on CUDA 9 and GCC 5.5

Author: reyoung

cmake/external/warpctc.cmake

@@ -52,6 +52,7 @@ ExternalProject_Add(
  -DWITH_TORCH=OFF
  -DCMAKE_DISABLE_FIND_PACKAGE_Torch=ON
  -DBUILD_SHARED=ON
+ -DBUILD_TESTS=OFF
  -DCMAKE_POSITION_INDEPENDENT_CODE=ON
  -DCMAKE_BUILD_TYPE=${THIRD_PARTY_BUILD_TYPE}
  ${EXTERNAL_OPTIONAL_ARGS}