Build C code in seeder with PIE_FLAGS

Summary:
This avoids a relocation error for libbitcoin_seeder_a-dns.o when building
the seeder shared library on some Debian environments (rare).

The build problem was observed on Debian 8 x86_64 derivate with gcc 7.1.0:

/usr/bin/ld: libbitcoin_seeder.a(libbitcoin_seeder_a-dns.o): relocation R_X86_64_32 against '.rodata' can not be used when making a shared object; recompile with -fPIC

Depends D581 .

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D582