`JSONCPP_HAS_STRING_VIEW` is not defined during Meson build

[mgorny]

Describe the bug
1.9.7 release added some code guarded by JSONCPP_HAS_STRING_VIEW. However, the relevant check is only present in CMake files and so the define is never set when building via meson.

To Reproduce
https://github.com/search?q=repo%3Aopen-source-parsers%2Fjsoncpp%20jsoncpp_has_string_view&type=code

Expected behavior
JSONCPP_HAS_STRING_VIEW in meson.build.

Desktop (please complete the following information):
n/a

Additional context
I was debugging build failures from CMake against new jsoncpp, with errors such as:

/usr/x86_64-pc-linux-gnu/binutils-bin/2.46.0/ld: cmScanDepFormat.cxx:(.text+0xc9a): undefined reference to `Json::Value::operator[](std::basic_string_view<char, std::char_traits<char> >) const'

Not sure if it's strictly related but looked like a bug.