As of Fri, 7 Aug 2015 01:44:49 -0400 (05:44 +0000) concepts support has been merged into gcc’s trunk.
Using a build from after that point, you can enable concepts support with the -std=c++1z flag.
As of gcc 6.1 (27-04-2016), concepts are enabled by -fconcepts as they are unlikely to be included in C++17, so they have been removed from -std=c++1z. (via @BruceAdams in comments below)