doc: how to unset Clear Linux' CFLAGS and CXXFLAGS

Clear Linux exports to all users a list of "aggressive" compiler and
linker flags. Zephyr's CMake build system will either warn or fail
because of these. Add one magic command that solves the issue.

More background information at https://superuser.com/a/1452523/111302



Signed-off-by: Marc Herbert <marc.herbert@intel.com>