PATCH to c-commonc.c, preliminary for PR/3865 fix

[Neil Booth]

Zack Weinberg wrote:-

> I believe it would be more efficient to handle these macros in
> cppmacro.c:_cpp_builtin_macro_text, taking their values from a
> structure filled in by c-common.c and passed into cpplib.  However,
> thinking about it a bit more, this is premature optimization; never
> mind.

Sometime in 3.4 I might improve this area a bit.  e.g. it is probably
worth having a cpp_define_bool (macroname, on) which defines macroname
to be 1 or 0.  We could avoid all the buffer pushing, lexing and popping
in this common case (>80%?)

Neil.