<sys/cdefs.h>: Fix the _Static_assert compatiblity macros.

Remove the typedef hack (which was like our CTASSERT()). Emacs was
choking on it because it is using _Static_assert like this (in verify.h):

#define _GL_VERIFY_TYPE(R, DIAGNOSTIC)  \
     struct {                           \
       _Static_assert (R, DIAGNOSTIC);  \
       int _gl_dummy;                   \
     }

GCC supports it since 4.6, so take that into account too.
make.1: correct a few style issues