Allow multiple gnulib generated include files to be combined.

Allow multiple gnulib generated include files to be combined.

* gnulib-tool (func_compute_include_guard_prefix): New function.
(func_emit_lib_Makefile_am, func_emit_tests_Makefile_am): Resolve also
${gl_include_guard_prefix} references.
(func_import, func_create_testdir): Invoke
func_compute_include_guard_prefix.
* lib/arpa_inet.in.h: Use the @GUARD_PREFIX@ placeholder.
* lib/ctype.in.h: Likewise.
* lib/dirent.in.h: Likewise.
* lib/errno.in.h: Likewise.
* lib/fcntl.in.h: Likewise.
* lib/float.in.h: Likewise.
* lib/getopt.in.h: Likewise.
* lib/iconv.in.h: Likewise.
* lib/langinfo.in.h: Likewise.
* lib/locale.in.h: Likewise.
* lib/math.in.h: Likewise.
* lib/netdb.in.h: Likewise.
* lib/netinet_in.in.h: Likewise.
* lib/poll.in.h: Likewise.
* lib/pthread.in.h: Likewise.
* lib/pty.in.h: Likewise.
* lib/sched.in.h: Likewise.
* lib/se-selinux.in.h: Likewise.
* lib/search.in.h: Likewise.
* lib/signal.in.h: Likewise.
* lib/spawn.in.h: Likewise.
* lib/stdarg.in.h: Likewise.
* lib/stddef.in.h: Likewise.
* lib/stdint.in.h: Likewise.
* lib/stdio.in.h: Likewise.
* lib/stdlib.in.h: Likewise.
* lib/string.in.h: Likewise.
* lib/strings.in.h: Likewise.
* lib/sys_file.in.h: Likewise.
* lib/sys_ioctl.in.h: Likewise.
* lib/sys_select.in.h: Likewise.
* lib/sys_socket.in.h: Likewise.
* lib/sys_stat.in.h: Likewise.
* lib/sys_time.in.h: Likewise.
* lib/sys_times.in.h: Likewise.
* lib/sys_uio.in.h: Likewise.
* lib/sys_utsname.in.h: Likewise.
* lib/sys_wait.in.h: Likewise.
* lib/sysexits.in.h: Likewise.
* lib/termios.in.h: Likewise.
* lib/time.in.h: Likewise.
* lib/unistd.in.h: Likewise.
* lib/wchar.in.h: Likewise.
* lib/wctype.in.h: Likewise.
* modules/arpa_inet (Makefile.am): Substitute @GUARD_PREFIX@.
* modules/ctype (Makefile.am): Likewise.
* modules/dirent (Makefile.am): Likewise.
* modules/errno (Makefile.am): Likewise.
* modules/fcntl-h (Makefile.am): Likewise.
* modules/float (Makefile.am): Likewise.
* modules/getopt-posix (Makefile.am): Likewise.
* modules/iconv-h (Makefile.am): Likewise.
* modules/langinfo (Makefile.am): Likewise.
* modules/locale (Makefile.am): Likewise.
* modules/math (Makefile.am): Likewise.
* modules/netdb (Makefile.am): Likewise.
* modules/netinet_in (Makefile.am): Likewise.
* modules/poll-h (Makefile.am): Likewise.
* modules/pthread (Makefile.am): Likewise.
* modules/pty (Makefile.am): Likewise.
* modules/sched (Makefile.am): Likewise.
* modules/search (Makefile.am): Likewise.
* modules/selinux-h (Makefile.am): Likewise.
* modules/signal (Makefile.am): Likewise.
* modules/spawn (Makefile.am): Likewise.
* modules/stdarg (Makefile.am): Likewise.
* modules/stddef (Makefile.am): Likewise.
* modules/stdint (Makefile.am): Likewise.
* modules/stdio (Makefile.am): Likewise.
* modules/stdlib (Makefile.am): Likewise.
* modules/string (Makefile.am): Likewise.
* modules/strings (Makefile.am): Likewise.
* modules/sys_file (Makefile.am): Likewise.
* modules/sys_ioctl (Makefile.am): Likewise.
* modules/sys_select (Makefile.am): Likewise.
* modules/sys_socket (Makefile.am): Likewise.
* modules/sys_stat (Makefile.am): Likewise.
* modules/sys_time (Makefile.am): Likewise.
* modules/sys_times (Makefile.am): Likewise.
* modules/sys_uio (Makefile.am): Likewise.
* modules/sys_utsname (Makefile.am): Likewise.
* modules/sys_wait (Makefile.am): Likewise.
* modules/sysexits (Makefile.am): Likewise.
* modules/termios (Makefile.am): Likewise.
* modules/time (Makefile.am): Likewise.
* modules/unistd (Makefile.am): Likewise.
* modules/wchar (Makefile.am): Likewise.
* modules/wctype-h (Makefile.am): Likewise.
* modules/assert-h (Makefile.am): Replace _GL_VERIFY_H specially.