Add configure check for _Static_assert()

Commit 8efede20ef ("Use _Static_assert") introduced the usage of
_Static_assert(). However, _Static_assert() is a fairly new thing,
since it was introduced only in gcc 4.6. In order to support older
compilers, this patch adds a configure.in test that checks whether
_Static_assert() is usable or not, and adjust the behavior of the
assert_cc() macro accordingly.
build: remove check for typeof

It's used in so many places without checking, that's really pointless to
check for it in macro.h.

Also remove AC_C_TYPEOF from configure.ac since we don't use -ansi.