net-mgmt/flow-tools: respect CFLAGS and unbreak with gcc5 or later

../lib/libft.a(ftfil.o): In function `resolve_primitives':
ftfil.c:(.text+0x12bb): undefined reference to `eval_match_src_as'
ftfil.c:(.text+0x13e7): undefined reference to `eval_match_ip_prot'
ftfil.c:(.text+0x1432): undefined reference to `eval_match_ip_src_prefix_len'
[...]

PR: 216707
Reported by: antoine (via exp-run)