[SOLVED] Any way to specify the required IDE version for Boards Manager install?

No, not as yet (through lib manager). You can use #error to notify in code of an incompatible version, and what version to use.

I use C++11 for some features in my PrintEx lib, I've just disabled these in code until 1.6.6.

You can check the define ARDUINO is greater than, or equal to 10606