Makefile: give CFLAGS to linker
Makefile: give CFLAGS to linker

ACX_PTHREAD will put -pthread to CFLAGS.  This needs also to
reach linker.

Reported-by: Pierre-Emmanuel André

kvap/pgbouncer,wurenny/pgbouncer-x2,jaiminpan/pgbouncer,dbaxa/pgbouncer,kvap/pgbouncer,wurenny/pgbouncer-x2,dbaxa/pgbouncer,digoal/pgbouncer-x2,kvap/pgbouncer,kvap/pgbouncer,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,digoal/pgbouncer-x2,jaiminpan/pgbouncer,dbaxa/pgbouncer,jaiminpan/pgbouncer,dbaxa/pgbouncer,digoal/pgbouncer-x2,jaiminpan/pgbouncer,wurenny/pgbouncer-x2