doc: rewrite subsecond NEWS entry, sanity.m4 "make" testing wrong, etc.

doc: rewrite subsecond NEWS entry, sanity.m4 "make" testing wrong, etc.

* m4/sanity.m4 (_AM_FILESYSTEM_TIMESTAMP_RESOLUTION): mention
that testing of "make" is wrong here; but $(MAKE) is not available?
Do some special pleading that it won't matter in practice; we'll see.
https://lists.gnu.org/archive/html/automake/2024-06/msg00051.html
(AM_SANITY_CHECK): use $[]* instead of $[*]2 to avoid
the confusing apparent (but not) use of shell arrays; the square
brackets are parsed off by m4.
* lib/am/check.am (output_system_information): en passant, mention
that $(AWK) should always be defined since AC_PROG_AWK is part of
AM_INIT_AUTOMAKE nowadays.
* NEWS: rewrite subsecond NEWS entry:workaround, more references.