Fix conflict between two instantiations of module 'unistd'.

Fix conflict between two instantiations of module 'unistd'.

* gnulib-tool (func_emit_autoconf_snippet): Substitute
${include_guard_prefix} also in the autoconf snippet.
* m4/getopt.m4 (gl_FUNC_GETOPT_POSIX): Don't set GNULIB_UNISTD_H_GETOPT.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Don't initialize
GNULIB_UNISTD_H_GETOPT.
* modules/getopt-posix (configure.ac): Set the
GNULIB_${gl_include_guard_prefix}_UNISTD_H_GETOPT variable.
* modules/getopt-gnu (configure.ac): Likewise.
* modules/unistd (Makefile.am): Change the substitution value of
GNULIB_UNISTD_H_GETOPT to depend on the include guard prefix.
Reported by Simon Josefsson.