revert the latest change

./configure fails during the "BUILD_LIBPCAP=true BUILD_IPV6=true"
scenarios (both GCC and Clang):

Makefile:8411: warning: overriding commands for target `configure'
Makefile:455: warning: ignoring old commands for target `configure'
Makefile:8416: warning: overriding commands for target `stamp-h.in'
Makefile:460: warning: ignoring old commands for target `stamp-h.in'
Makefile:8421: warning: overriding commands for target `stamp-h'
Makefile:465: warning: ignoring old commands for target `stamp-h'
Makefile:8424: warning: overriding commands for target `Makefile'
Makefile:468: warning: ignoring old commands for target `Makefile'
Makefile:8427: warning: overriding commands for target `config.status'
Makefile:471: warning: ignoring old commands for target `config.status'
[...]
./config.status: line 1103: syntax error near unexpected token `done'
./config.status: line 1103: `done # for ac_tag'

I don't have the expertise to resolve it well right now, keep Travis CI
working meanwihle.