Now that the context switch code is red zone safe, allow the compiler
to use it.

Causes an early failure for some reason.  Disable until root cause is found.

 'make conf-preempt=0'

->

 'g++ ... -DCONF_preempt=0'

Use #if, not #ifdef in sources to reduce errors due to misspellings.

Easier to manage different configurations this way.