configure: check typeof on C++ compilers

C compilers can have GNU extensions to support typeof in C code, but
some C++ compilers like clang are removing the builtin since decltype
can be used in C++ without the constraints from typeof. Decltype is not
100% equivalent for this reason: references will be kept in the returned
type.

The check in m4/typeof.m4 comes from graydon/monotone and dovecot/core
and was slightly modified to namespace the define for C++ code.