build: Add BUILD_OBJEXT sanity check

configure.ac

@@ -78,6 +78,15 @@ AC_PROG_LN_S
 AC_PROG_AWK
 AC_PROG_INSTALL
 
+AS_IF([test "x${BUILD_OBJEXT-}" = x],
+  [AS_IF([test "$cross_compiling" = no],
+    [BUILD_EXEEXT="$EXEEXT"
+    BUILD_OBJEXT="$OBJEXT"],
+    [BUILD_EXEEXT="${ac_cv_build_exeext-}"
+    BUILD_OBJEXT="${ac_cv_build_objext-}"])])
+AS_IF([test "x${BUILD_OBJEXT-}" = x && test "x${enable_bootstrap-yes}" = xyes],
+  [AC_MSG_ERROR([BUILD_OBJEXT is invalid; please regenerate 'configure' with a newer ax_prog_cc_for_build.m4 (serial 22 or later) or configure with '--disable-bootstrap'])])
+
 pkgconfigdir=${libdir}/pkgconfig
 AC_SUBST(pkgconfigdir)