build: Silence warning about deprecated INCLUDES variable

Since automake 1.12.4, the warnings are issued on running automake:
    warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

Avoid INCLUDES and move these flags into thermald_CPPFLAGS
build: Explicitly enable subdir-objects option

Automake 1.14 throws a warning when source files are in a sub directory
and the 'subdir-objecs' option is not enabled.

News: https://lists.gnu.org/archive/html/automake/2013-06/msg00040.html