build/cmake: support c++ other than 17

CMakeLists.txt

@@ -59,7 +59,9 @@ if (DOXYGEN_FOUND)
     option (OPENDHT_DOCUMENTATION "Create and install the HTML based API documentation (requires Doxygen)" ${DOXYGEN_FOUND})
 endif ()
 # Build flags
-set (CMAKE_CXX_STANDARD 17)
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set (CMAKE_CXX_STANDARD 17)
+endif()
 set (CMAKE_CXX_STANDARD_REQUIRED on)
 if(MSVC AND NOT BUILD_SHARED_LIBS AND NOT DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
     set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")