some queston about make

[loody]

Dear all:
I have some questions about make usage:
1. when passing command to shell why we have to add ";" at each end of command.
Is it the rule define in make or in shell?
example like below:
do_test:
 libs='-L. ${SHLIBDEPS}'; for i in ${SHLIBDIRS}; do \
                $(CLEARENV) && $(MAKE) -f Makefile.shared -e $(BUILDENV) \
                        link_a.$(SHLIB_TARGET); \
                libs="-l$$i $$libs"; \
        done

2. why sometimes we use "{}" to embrace variable instead of "()"
Is that due to we have to pass it to shell?

example like below:
ifeq (0,${MAKELEVEL})
whoami := $(shell whoami)
host-type := $(shell arch)
MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
endif

appreciate your help,
miloody