The problem is that although AIX implements bzero, its prototype
is not declared by default. Since AC_CHECK_FUNC(bzero) succeeds
even though a prototype is not declared, this breaks compilation
in C++ files where a prototype is required.

The solution is to only use bzero if a prototype is also declared.

configure.in:
  Check if bzero is declared. No need to specify the includes,
  unisted.h and strings.h are already part of AC_INCLUDES_DEFAULT.