jsoncpp 1.9.7 breaks c++11 support

[jpalus]

Describe the bug
After #1584 jsoncpp supports either C++11, if jsoncpp was compiled with C++11 standard or C++17 and later (no C++11) if compiled with ie C++17. That's because symbols in library are exclusive:

jsoncpp/src/lib_json/json_value.cpp

Lines 1193 to 1224 in 3455302

	#ifdef JSONCPP_HAS_STRING_VIEW
	const Value& Value::operator[](std::string_view key) const {
	Value const* found = find(key.data(), key.data() + key.length());
	if (!found)
	return nullSingleton();
	return *found;
	}
	Value& Value::operator[](std::string_view key) {
	return resolveReference(key.data(), key.data() + key.length());
	}
	#else
	const Value& Value::operator[](const char* key) const {
	Value const* found = find(key, key + strlen(key));
	if (!found)
	return nullSingleton();
	return *found;
	}
	Value const& Value::operator[](const String& key) const {
	Value const* found = find(key);
	if (!found)
	return nullSingleton();
	return *found;
	}
	Value& Value::operator[](const char* key) {
	return resolveReference(key, key + strlen(key));
	}
	Value& Value::operator[](const String& key) {
	return resolveReference(key.data(), key.data() + key.length());
	}
	#endif

To Reproduce
Steps to reproduce the behavior:

1. Compile jsoncpp with ie C++20
2. Try to compile and link following code with C++11

#include <json/json.h>
int main() {
  Json::Value v;
  v["test"];
}

3. Build fails on linking:

test.c:(.text+0x28): undefined reference to `Json::Value::operator[](char const*)'

Expected behavior
Build succeeds.

[evan-dynamsoft]

I have closed the issue I opened (#1670).
In addition to requiring that interfaces meant for C++11 must be included, I also want the header definitions and the library implementation to be consistent.

In value.h:

#ifndef JSONCPP_HAS_STRING_VIEW
#if __cplusplus >= 201703L
#define JSONCPP_HAS_STRING_VIEW 1
#endif
#endif

When JsonCpp is compiled under C++11, the implementation does not contain the interfaces wrapped by #ifdef JSONCPP_HAS_STRING_VIEW.
However, when the same header is included in a C++20 project, those interfaces become available.
This inconsistency is unreasonable.