Newer versions of GCC (8.1.0 in my case) default to `-std=gnu11` (I believe), which expects `inline` to have an external definition. The `Makefile` (I ran it with `NO_ARDUINO` set) doesn't specify any C_FLAGS. Therefore gcc uses the new default value, instead of the old C99 standard. In this case you'll get a linker error for `getIsrTable` not being defined. This change allows the code to successfully compile with new versions of gcc. This change works with the old default std of gcc. For much older versions of gcc it might be better to explicitly set the `std` in `CFLAGS` within the `Makefile`.

Use ARM's unsigned saturate instruction to limit the value, to be written to DAC, to 11 bits (range 0 to 4095).

fixes #266

https://forum.pjrc.com/threads/49442-Teensy-3-5-FlightSim-Float-problem

Sorry, this is a backwards incompatible change.  :(  Painful but needed long-term...