bump cmake requirement to 3.1, add INTERFACE header declaration
bump cmake requirement to 3.1, add INTERFACE header declaration

Formlabs/tbb,wjakob/tbb,wjakob/tbb,Formlabs/tbb,Formlabs/tbb,Formlabs/tbb,wjakob/tbb,Formlabs/tbb,wjakob/tbb,wjakob/tbb,Formlabs/tbb,wjakob/tbb,Formlabs/tbb,wjakob/tbb