(blind make headers from specifications)

`configure` should not be bothered with defining macros for generating
`transitioning.ml`. This belongs to `Makefile.generating`

`Stack.fold` didn't exist back then 

Also update comments of `transitioning.mli` to reflect reality

- Debug_category is now only used for its side-effects, the only
  relevant state is in Log.messages
- allow to use `String.split_on_char` in `Transitioning`

if CPP is set at runtime, we do not assume that it has options to keep
comments. Moreover, if CPP + default args (`-C -I.`) is equal to the
default computed at compile time, we use the appropriate flags for
`is_gnu_like` and `keep_comments`

Otherwise, plug-ins compiled in internal mode that do some additional
C-related checks are not 

- Extend machdep information to include required CPP arguments (p.ex. -m32);
- Check supported architecture options during configure;
- Automatically apply options according to chosen machdep.

Note: this may generate extra warnings when using old versions of GCC or
compilers that do not support options -m32/-m64 and similar.