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

[Paul Eggert]

Florian Weimer wrote:

> #ifndef _GL_INLINE_HEADER_BEGIN
>  #error "Please include config.h first."
> #endif
>
> This will result in an #error if the installed header <argp.h> is used by
> applications.

Then let's remove those three lines. They are not essential for Gnulib, and 
staying in sync with glibc is more important than the extra error-checking they 
might provide for Gnulib users.