Fall back on busybox awk

* lib/autoconf/programs.m4 (AC_PROG_AWK):
Fall back 'busybox awk' of none of the other awks work.

NEWS

@@ -28,6 +28,8 @@ GNU Autoconf NEWS - User visible changes.
   The autom4te, autoscan and ifnames programs now recognize the two
   preprocessor directives, which were introduced in C23 and C++23.
 
+*** AC_PROG_AWK now also checks for busybox awk.
+
 *** AC_USE_SYSTEM_EXTENSIONS now defines _COSMO_SOURCE for Cosmopolitan Libc.
 
 ** Notable bug fixes

doc/autoconf.texi

@@ -4162,14 +4162,19 @@ otherwise to @samp{:} (do nothing).
 @acindex{PROG_AWK}
 @ovindex AWK
 @caindex prog_AWK
-Check for @code{gawk}, @code{mawk}, @code{nawk}, and @code{awk}, in that
+Check for the commands @command{gawk}, @command{mawk},
+@command{nawk}, @command{awk}, and @code{busybox awk}, in that
 order, and set output variable @code{AWK} to the first one that is found.
-It tries @code{gawk} first because that is reported to be the
+Try @code{gawk} first because that is reported to be the
 best implementation.  The result can be overridden by setting the
 variable @code{AWK} or the cache variable @code{ac_cv_prog_AWK}.
 
-Using this macro is sufficient to avoid the pitfalls of traditional
-@command{awk} (@pxref{awk, , Limitations of Usual Tools}).
+Using this macro avoids some pitfalls of older systems that have both
+traditional @command{awk} (@pxref{awk, , Limitations of Usual Tools})
+and better alternatives like @command{gawk}.
+It also can help port to stripped-down systems that have only
+@code{busybox awk}, although these systems could well have problems
+with other build components that use plain @command{awk}.
 @end defmac
 
 @defmac AC_PROG_GREP

lib/autoconf/programs.m4

@@ -357,7 +357,7 @@ AN_PROGRAM([gawk], [AC_PROG_AWK])
 AN_PROGRAM([mawk], [AC_PROG_AWK])
 AN_PROGRAM([nawk], [AC_PROG_AWK])
 AC_DEFUN([AC_PROG_AWK],
-[AC_CHECK_PROGS(AWK, gawk mawk nawk awk, )])
+[AC_CHECK_PROGS([AWK], [gawk mawk nawk awk 'busybox awk'])])
 
 
 # AC_PROG_EGREP