tools thermal tmon: Allow overriding CFLAGS assignments

So that the user can provide, e.g. distro package alternative values.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Herton Krzesinski  <herton@redhat.com>
Cc: Markus Mayer <mmayer@broadcom.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Link: http://lkml.kernel.org/r/20181212102537.25902-3-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>