sys_un: Avoid compilation error in C++ <string.h> on glibc systems.
* m4/sys_un_h.m4 (gl_SYS_UN_H): Require AC_CANONICAL_HOST. Set
GL_GENERATE_SYS_UN_H to true on glibc systems. Invoke
gl_CHECK_NEXT_HEADERS. Initialize HAVE_SYS_UN_H.
* lib/sys_un.in.h: Use include_next and a split double-inclusion guard.
In C++ mode, include <stddef.h> and <string.h> first.
* modules/sys_un (Depends-on): Add include_next.
(Makefile.am): Substitute INCLUDE_NEXT, PRAGMA_SYSTEM_HEADER,
PRAGMA_COLUMNS, NEXT_SYS_UN_H, HAVE_SYS_UN_H. Arrange to remove 'sys'
directory at "make mostlyclean".