AM_SILENT_RULES issue with automake 1.17

automake 1.17 available for instance on macOS with homebrew shows a regression bug triggered by this in confingure.ac:

m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])dnl be backward compatible
AC_CONFIG_HEADERS([config.h])

the expansion of AM_SILENT_RULES is not followed by a newline so is mixed with the config.h stuff.

The bug was reported to automake team as https://debbugs.gnu.org/cgi/bugreport.cgi?bug=72267 which suggests:

To this:

m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])
])dnl
AC_CONFIG_HEADERS([config.h])

Alternately just leave it the same and remove the dnl instead, and don't worry about the extra newline with some automake versions.