[PLUGINS] Add 'extern "C"' wrappers to several header files

[Ian Lance Taylor]

Duncan Sands <baldrick@free.fr> writes:

> I am writing a gcc plugin in C++.  In order to use gcc's functions
> and objects from C++, they need to be declared as having C linkage.
> The attached patch wraps the body of various gcc header files with
>   #ifdef __cplusplus
>   extern "C" {
>   #endif /* __cplusplus */
>   ...
>   #ifdef __cplusplus
>   }
>   #endif /* __cplusplus */

This approach fits poorly with --enable-build-with-cxx.

> I made no attempt to do this systematically: these are the headers
> I needed to modify for my plugin to link.  At first sight it might
> appear that there is no need to modify gcc, since the plugin can
> include header files like this:
>   extern "C" {
>   #include "flags.h"
>   }
> But in fact it cannot: some gcc header files include C++ aware
> system headers.  For example, flags.h includes real.h, and real.h
> includes gmp.h, and gmp.h includes C++ system headers if compiled
> from C++.  The result is that if I wrap the inclusion of flags.h in
> an 'extern "C"' wrapper then C++ code ends up with C linkage, and
> compilation fails (templates particularly don't like having C linkage).

One way to avoid this issue is to #include <gmp.h> before including any
gcc header files.

I agree that the current situation is less than idea but I don't think
that using extern "C" in the gcc header files is the right approach.

Ian