configurator: regression in handling expressions for C imports

[avsm]

The fix for importing negative numbers in header files (#1334) added a dependency on the expression passed in also working with cpp. This prevents expressions such as sizeof(char) from being evaluated, as it leads to this error:

 | <...>/test.c:27:5: error: function-like macro 'sizeof' is not defined
 | #if sizeof(char) >= 0
 |     ^
 | 1 error generated.

I'm not sure what the best way to handle this is without knowing ahead of time if the value is possibly signed or not. We could distinguish C_define.Type to have a Uint that will not need the preprocess change above...

(This issue comes up when porting ctypes to dune)

[emillon]

Closed by #1726