add define.mk and override_define.mk

define.mk
% make
echo BEGIN echo foo
BEGIN echo foo
echo xxx END
xxx END
% ../../kati
*kati*: parseAssign bar = xxx =
*kati*: define two-lines
*kati*: echo foo
*kati*: echo $(bar)
*kati*: endef
*kati*: bar=xxx
*kati*: test:
*kati*:         echo BEGIN $(two-lines) END
Makefile:17: *** missing separator.

override_define.mk
% make
echo BEGIN echo foo
BEGIN echo foo
echo xxx END
xxx END
% ../../kati
*kati*: parseAssign bar = xxx =
*kati*: override define two-lines
*kati*: echo foo
*kati*: echo $(bar)
*kati*: endef
*kati*: bar=xxx
*kati*: test:
*kati*:         echo BEGIN $(two-lines) END
Makefile:5: *** missing separator.
2 files changed