[PATCH] argp: Do not override GCC keywords with macros [BZ #16907]

[Paul Eggert]

Florian Weimer wrote:
> This commit does not sync with gnulib because gnulib has started to use
> _GL_* macros in the header file, which are arguably in the gnulib
> implementation space and not suitable for an installed glibc header
> file.

I'd rather keep the files in sync if possible. How about if we rename the 
affected macro from _GL_ATTRIBUTE_FORMAT to __attribute_format__ (or whatever 
other name you like that is in the glibc space? That way, Gnulib can stay in 
sync better.

Similarly, the files shared with Gnulib can define __BEGIN_DECLS and __END_DECLS 
when __BEGIN_DECL is not already defined.