Update ax_check_compile_flag.m4

m4/ax_check_compile_flag.m4

@@ -55,10 +55,10 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 3
+#serial 4
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
-[AC_PREREQ(2.59)dnl for _AC_LANG_PREFIX
+[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 $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
@@ -67,7 +67,7 @@ AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
     [AS_VAR_SET(CACHEVAR,[yes])],
     [AS_VAR_SET(CACHEVAR,[no])])
   _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
-AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes],
+AS_VAR_IF(CACHEVAR,yes,
   [m4_default([$2], :)],
   [m4_default([$3], :)])
 AS_VAR_POPDEF([CACHEVAR])dnl