you don't need any special tricks to build tbb for knl, just build it for intel64 architecture. but for modules or your app that use tbb headers you can use knl specific keys like -xHost if you build on knl system or -xMIC-AVX512 if you build on different system.
By default the Makefile just uses the gcc I have on the system (4.9.3) in my case.
To clarify, I use the default gcc to make the tbb library, then use the KNL specific flags on my application?
Above instructions are for current (2016) Intel compiler. For gcc 4.9.3 instructions are pretty the same, use default 'make tbb tbbmalloc arch=intel64' to build the tbb library and then -mavx512f gcc flag to build your application.