Allow overriding CMAKE_CXX_COMPILER (#436)

Using set as-is doesn't allow the user to set their own rocm path.
This is useful for community packagers or debugging.

Signed-off-by: Jeremy Newton <Jeremy.Newton@amd.com>

[ROCm/rocdecode commit: 07ecb5e7d6]

projects/rocdecode/samples/videoDecodePerf/CMakeLists.txt

@@ -36,7 +36,7 @@ endif()
 
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/../../cmake)
 list(APPEND CMAKE_PREFIX_PATH ${ROCM_PATH}/hip ${ROCM_PATH})
-set(CMAKE_CXX_COMPILER ${ROCM_PATH}/lib/llvm/bin/clang++)
+set(CMAKE_CXX_COMPILER ${ROCM_PATH}/lib/llvm/bin/clang++ CACHE STRING "ROCm Compiler path")
 
 # rocDecode sample build type
 set(DEFAULT_BUILD_TYPE "Release")