Add support for c++11 (i.e. for centos 7)

[al-zatv]

1) Add make_unique() function, which is the only missing piece to make kaldi-serve on c++11 2) switch -std=c++17 to -std=c++11 in Makefile

[pskrunner14]

Hi sorry about the late response, but something similar has been done in the recent library v1 release (supports C++11 std now)

[al-zatv]

Great!