Since we added mini-gmp.c as a part of libbitcoin_server_a_SOURCES,
-fPIC is required to avoid the linkage error:

/usr/bin/ld: libbitcoin_server.a(libbitcoin_server_a-mini-gmp.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
libbitcoin_server.a: error adding symbols: Bad value
collect2: error: ld returned 1 exit status

TODO: In future we should totally (!) get rid of mini-gmp.c, for example,
same way as i did here - bf14e82 ,
bcz actually we using only two functions from this mini-gmp.c - it's
bitcoin_base58decode and bitcoin_base58encode, which are perfectly
could be replaced with C++ analogs DecodeBase58, EncodeBase58 from
base58.h