Makefile: add multi-line target, target variable value, target separator and...

Makefile: add multi-line target, target variable value, target separator and keyword ; fix define and escaped rule

• multi-line target

target1 \
target2: ....

• target-specific variable value

prog: private EXTRA_CFLAGS = -L/usr/local/lib
prog: CFLAGS = -g
 #    ^ variable
 #            ^ value

• target separator

prog:
prog::
prog&:
prog&::
 #  ^ TargetSeparator style

• fix escaped rule

prog:
	echo bla \
   bla
 #^ no tab is ok

BUG: 470920