test compiler for c++17 support

config.m4

@@ -39,11 +39,15 @@ if test "$PHP_V8JS" != "no"; then
 
 
   AC_CACHE_CHECK(for C standard version, ac_cv_v8_cstd, [
-    ac_cv_v8_cstd="c++14"
+    ac_cv_v8_cstd="c++17"
     old_CPPFLAGS=$CPPFLAGS
     AC_LANG_PUSH([C++])
     CPPFLAGS="-std="$ac_cv_v8_cstd
-    AC_RUN_IFELSE([AC_LANG_SOURCE([[int main() { return 0; }]])],[],[ac_cv_v8_cstd="c++1y"],[])
+    AC_RUN_IFELSE([AC_LANG_SOURCE([[int main() { return 0; }]])],[],[
+      ac_cv_v8_cstd="c++14"
+      CPPFLAGS="-std="$ac_cv_v8_cstd
+      AC_RUN_IFELSE([AC_LANG_SOURCE([[int main() { return 0; }]])],[],[ ac_cv_v8_cstd="c++1y" ],[])
+    ],[])
     AC_LANG_POP([C++])
     CPPFLAGS=$old_CPPFLAGS
   ]);