fix: autoreconf fails due to .m4 files added but not installed

[Jim Meyering]

A few days ago I was preparing to release grep and wanted to make sure
it'd work with the latest automake and autoconf, so I built and installed
each from master. That exposed these errors when bootstrapping grep. I
think it was a prerequisite to have run grep's "make maintainer-clean":

  configure.ac:41: warning: _AM_PROG_RM_F is m4_require'd but not\
    m4_defun'd
  configure.ac:41: warning: _AM_PROG_XARGS_N is m4_require'd but not\
    m4_defun'd
  configure:5058: error: possibly undefined macro: _AM_PROG_RM_F
  configure:5059: error: possibly undefined macro: _AM_PROG_XARGS_N

I've just pushed the attached fix.
Introduced in v1.16.5-46-g38da1d906 and v1.16.5-45-g3099097d7
so this doesn't need a NEWS update.

Mike, can you add an automake test that would have caught this?

am-missing-m4-files.diff
Description: Text Data