Warn if PUGIXML_STRING_VIEW is set without CMAKE_CXX_STANDARD (#638)

Even if the compiler supports C++17, we define CMAKE_CXX_STANDARD as 11
by default which implicitly disables string_view support; for now warn
in this case.

CMakeLists.txt

@@ -48,7 +48,7 @@ option(PUGIXML_INSTALL "Enable installation rules" ON)
 option(PUGIXML_NO_XPATH "Disable XPath" OFF)
 option(PUGIXML_NO_STL "Disable STL" OFF)
 option(PUGIXML_NO_EXCEPTIONS "Disable Exceptions" OFF)
-option(PUGIXML_STRING_VIEW "Enable std::string_view overloads" OFF) # requires C++17 and for PUGI_NO_STL to be OFF
+option(PUGIXML_STRING_VIEW "Enable std::string_view overloads" OFF) # requires C++17 and for PUGIXML_NO_STL to be OFF
 mark_as_advanced(PUGIXML_NO_XPATH PUGIXML_NO_STL PUGIXML_NO_EXCEPTIONS PUGIXML_STRING_VIEW)
 
 set(PUGIXML_PUBLIC_DEFINITIONS
@@ -66,6 +66,10 @@ if (NOT DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
     MultiThreaded$<$<CONFIG:Debug>:Debug>$<$<NOT:$<BOOL:${PUGIXML_STATIC_CRT}>>:DLL>)
 endif()
 
+if (PUGIXML_STRING_VIEW AND (NOT DEFINED CMAKE_CXX_STANDARD OR CMAKE_CXX_STANDARD LESS 17))
+  message(WARNING "PUGIXML_STRING_VIEW requires CMAKE_CXX_STANDARD to be set to 17 or later")
+endif()
+
 if (NOT DEFINED CMAKE_CXX_STANDARD_REQUIRED)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
 endif()