Add configure option for c++17

Author: sipa

configure.ac

@@ -61,8 +61,20 @@ case $host in
      lt_cv_deplibs_check_method="pass_all"
   ;;
 esac
-dnl Require C++11 compiler (no GNU extensions)
-AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory])
+
+AC_ARG_ENABLE([c++17],
+  [AS_HELP_STRING([--enable-c++17],
+  [enable compilation in c++17 mode (disabled by default)])],
+  [use_cxx17=$enableval],
+  [use_cxx17=no])
+
+dnl Require C++11 or C++17 compiler (no GNU extensions)
+if test "x$use_cxx17" = xyes; then
+  AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
+else
+  AX_CXX_COMPILE_STDCXX([11], [noext], [mandatory])
+fi
+
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC