build: avoid getifaddrs when unavailable

Summary:

These changes from @theuni help building when targeting platforms that
don't always have getifaddrs available like Android < 24

Backport of core PR14127
https://github.com/bitcoin/bitcoin/pull/14127/files

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3824