cares-functions.m4: update thread-safeness detection of getaddrinfo()

cares-functions.m4: update thread-safeness detection of getaddrinfo()

Take in account that POSIX standard Issue 7 drops h_errno support. Now, we also
consider getaddrinfo() to be thread-safe when (_POSIX_C_SOURCE >= 200809L) or
(_XOPEN_SOURCE >= 700) independently of whether h_errno exists or not.

Take in account that h_errno might be a modifiable lvalue not defined as
a C preprocessor macro.