Update AX_CHECK_COMPILE_FLAG to serial 7 (#14217)

This syncs the file with upstream:
https://github.com/autoconf-archive/autoconf-archive/blob/master/m4/ax_check_compile_flag.m4

Author: petk

build/ax_check_compile_flag.m4

@@ -34,12 +34,12 @@
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 6
+#serial 7
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 [AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
 AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
-AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+AC_CACHE_CHECK([whether the _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
   _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
   AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],