glob: Don't compile replacements on recent glibc systems.
* lib/glob.in.h: Use the usual idiom for the double-inclusion guard. If
REPLACE_GLOB is 0, include the system's <glob.h> and use
_GL_CXXALIAS_SYS.
* m4/glob.m4 (gl_GLOB): Set REPLACE_GLOB instead of GLOB_H. Accept
_GNU_GLOB_INTERFACE_VERSION 2 as well. Delete the file conf$$-globtest
inside the AC_RUN_IFELSE block. Remove GL_GENERATE_GLOB_H conditional.
* modules/glob (Dependencies): Test REPLACE_GLOB instead of GLOB_H.
Remove snippet/warn-on-use.
(configure.ac): Test REPLACE_GLOB instead of GLOB_H.
(Makefile.am): Create glob.h always. Update list of substitutions in
glob.h. Don't depend on $(WARN_ON_USE_H).