Align offline compilation with cl21_trunk (#400)

Add a special case for the Khronos compiler and always use it for
OpenCL C++, otherwise use the same logic as on the cl21_trunk
branch.

The CMake option to pass the path to the khronos compiler has
been renamed to KHRONOS_OFFLINE_COMPILER_OPTIONS and is no longer
mandatory.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>

.travis.yml

@@ -24,7 +24,6 @@ script:
   - cmake -DCL_INCLUDE_DIR=${TOP}/OpenCL-Headers
           -DCL_LIB_DIR=${TOP}/OpenCL-ICD-Loader/build
           -DCL_LIBCLCXX_DIR=${TOP}/libclcxx
-          -DCL_OFFLINE_COMPILER=/dummy/path/to/compiler
           -DCMAKE_RUNTIME_OUTPUT_DIRECTORY=./bin
           -DOPENCL_LIBRARIES="-lOpenCL -lpthread"
           ..