add a directive to disable SwitchTransform

SwitchTransform is unable to detect all cases of duplicate values, which
result in errors at compile time in the generated code.
fix accidental removal of a line
rename "switchcase_transform" to "use_switch"
better logic for C compile failure support in runtests.py
Merge pull request #287 from gandalf013/switch_transform

add a directive to disable SwitchTransform