Patch hipcc environment variables in ROCm 3.X

In ROCm 3.0 and 3.1, environment variables for hipcc and hcc are
overriden by incorrect paths (espressomd/docker#156). This causes
CMake to generate an incorrect linking command for EspressoCore.so:
in `/opt/rocm/bin/hipcc_cmake_linker_helper /opt/rocm -fPIC ...`,
either path `/opt/rocm` is an empty string, or both the linker path
and path `/opt/rocm` are empty strings. Calling `find_package()`
twice with an overriden `HCC_PATH` fixes the linking command.

CMakeLists.txt

@@ -138,6 +138,9 @@ if(WITH_CUDA)
  set(ROCM_HOME "/opt/rocm" CACHE FILEPATH "Path to AMD ROCm")
  list(APPEND CMAKE_MODULE_PATH "${ROCM_HOME}/hip/cmake")
  find_package(HIP 1.5.18494 MODULE REQUIRED)
+ # patch HCC_PATH environment variable and reload HIP (for ROCm 3.0/3.1)
+ set(HCC_PATH "${HIP_ROOT_DIR}")
+ find_package(HIP 1.5.18494 MODULE REQUIRED)
  find_package(CUDACompilerHIP 6.0 REQUIRED)
  elseif (WITH_CUDA_COMPILER STREQUAL "clang")
  set(CMAKE_CUDA_STANDARD 8.0)

cmake/FindCUDACompilerHIP.cmake

@@ -34,8 +34,6 @@ string(REGEX
  CMAKE_CUDA_COMPILER_VERSION
  "${HIPCC_VERSION_STRING}")
 
-set(HCC_PATH "${HIP_ROOT_DIR}")
-
 list(APPEND HIP_HCC_FLAGS "-I${HIP_ROOT_DIR}/include -I${ROCM_HOME}/include -Wno-c99-designator -Wno-macro-redefined -Wno-duplicate-decl-specifier -std=c++${CMAKE_CXX_STANDARD}")
 list(APPEND HIP_HCC_FLAGS "-pedantic -Wall -Wextra -Wno-sign-compare -Wno-unused-function -Wno-unused-variable -Wno-unused-parameter -Wno-missing-braces -Wno-gnu-anonymous-struct -Wno-nested-anon-types -Wno-gnu-zero-variadic-macro-arguments")
 if(WARNINGS_ARE_ERRORS)