autoconf 2.71, AX_PROG_CC_FOR_BUILD and BUILD_*EXT

[Sven Verdoolaege]

Commit v2.69e-3-gc3853873 (Revert "AC_PROG_CC: define via AC_DEFUN_ONCE".
(#110350), Fri Dec 4 16:32:35 2020 -0500)
was apparently supposed to make AX_PROG_CC_FOR_BUILD work again,
but there appears to be a remaining issue.

With 

$ cat configure.ac 
AC_INIT([test], [0])
AC_CONFIG_MACRO_DIR([.])
AM_INIT_AUTOMAKE([foreign])

AX_PROG_CC_FOR_BUILD

AC_CONFIG_FILES(Makefile)
AC_OUTPUT

an empty Makefile.am and AX_PROG_CC_FOR_BUILD from
Autoconf Macro Archive, I find that BUILD_EXEEXT and BUILD_OBJEXT
are not getting set properly using autoconf 2.71.
In fact, I bisected it down to commit v2.69-18-g5fdd360b (AC_SUBST:
don't underquote the variable name, Wed Aug 15 21:54:53 2012 -0600).
The difference in the generated configure is as follows:

@@ -4622,7 +4622,7 @@ rm -f conftest conftest$ac_cv_build_exeext
 $as_echo "$ac_cv_build_exeext" >&6; }
 
 rm -f conftest.$ac_ext
-BUILD_EXEEXT=$ac_cv_build_exeext
+EXEEXT=$ac_cv_build_exeext
 ac_build_exeext=$BUILD_EXEEXT
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
@@ -4733,7 +4733,7 @@ rm -f conftest.$ac_cv_build_objext conftest.$ac_ext
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_build_objext" >&5
 $as_echo "$ac_cv_build_objext" >&6; }
-BUILD_OBJEXT=$ac_cv_build_objext
+OBJEXT=$ac_cv_build_objext
 ac_build_objext=$BUILD_OBJEXT
 
 ac_ext=c

Now, AX_PROG_CC_FOR_BUILD is calling _AC_COMPILER_EXEEXT and _AC_COMPILER_OBJEXT
which I guess it's not supposed to do, but it used to work...
Do you have any advice on how to make this work with recent versions
of autoconf?

Let me know if you need any further information.
Thanks,

skimo