setresuid and setresgid need _GNU_SOURCE which clashes with _BSD_SOURCE

--- a/configure.ac
+++ b/configure.ac
@@ -87,7 +87,7 @@
 AC_SEARCH_LIBS(socket, socket)
 AC_CHECK_LIB(pcap, pcap_open_live)
 
-AC_CHECK_FUNCS(closefrom daemon setresuid setreuid setresgid setgid strlcpy strlcat)
+AC_CHECK_FUNCS(closefrom daemon setreuid setgid strlcpy strlcat)
 
 AC_CHECK_TYPES([u_int64_t, int64_t, uint64_t, u_int32_t, int32_t, uint32_t])
 AC_CHECK_TYPES([u_int16_t, int16_t, uint16_t, u_int8_t, int8_t, uint8_t])