CMake improvements.

- Directed tests build hip_hcc as subdir.
- Support HCC_HOME env var to control which HCC to use for build
  hip_hcc.


[ROCm/hip commit: c4e1aba862]

projects/hip/include/hcc_detail/hip_hcc.h

@@ -96,7 +96,7 @@ static thread_local hipError_t tls_lastHipError = hipSuccess;
 
 
 // #include CPP files to produce one object file
-#define ONE_OBJECT_FILE 1
+//#define ONE_OBJECT_FILE 1
 
 
 // Compile support for trace markers that are displayed on CodeXL GUI at start/stop of each function boundary.