Use find_program() to set CMAKE_C_COMPILER and CMAKE_CXX_COMPILER.

Originally the variables was set to compiler name, without absolute path.
CMake 3.0 tries to find them in CMAKE_FIND_ROOT_PATH (since 
CMAKE_FIND_ROOT_PATH_MODE_PROGRAM was set to ONLY) and fails.

it tries to remove 'opencv_' prefix from 'name' variable, but
it creates new variable with the same name as module

Signed-off-by: Yan Wang <yan.wang@linux.intel.com>

it might be called from _OutputArray::setTo

ported from c105b729