chore: remove leftover CMake checks for std::string_view

Fixes #1669

This removes the final vestige of the JSONCPP_HAS_STRING_VIEW build system logic.

As of the previous commit (inlining std::string_view methods into value.h to fix ABI breaks), the library no longer relies on the build system (CMake or Meson) to check for and define JSONCPP_HAS_STRING_VIEW.

The header value.h automatically activates std::string_view overloads purely by checking the consumer`s __cplusplus >= 201703L. Since none of the actual std::string_view symbols are compiled into the .so / .a library anymore, Meson (and CMake) builds are identical regardless of whether string_view is supported by the compiler building the library.

src/lib_json/CMakeLists.txt

diff --git a/src/lib_json/CMakeLists.txt b/src/lib_json/CMakeLists.txt
index a0695e9..dd73af3 100644
--- a/src/lib_json/CMakeLists.txt
+++ b/src/lib_json/CMakeLists.txt
@@ -7,7 +7,6 @@
 include(CheckTypeSize)
 include(CheckStructHasMember)
 include(CheckCXXSymbolExists)
-include(CheckCXXSourceCompiles)
 
 check_include_file_cxx(clocale HAVE_CLOCALE)
 check_cxx_symbol_exists(localeconv clocale HAVE_LOCALECONV)
@@ -26,10 +25,7 @@
     endif()
 endif()
 
-check_cxx_source_compiles(
-    "#include <string_view>
-     int main() { std::string_view sv; return 0; }"
-    JSONCPP_HAS_STRING_VIEW)
+
 
 set(JSONCPP_INCLUDE_DIR ../../include)
 

src/lib_json/json_value.cpp

diff --git a/src/lib_json/json_value.cpp b/src/lib_json/json_value.cpp
index a8eb72d..5b4df37 100644
--- a/src/lib_json/json_value.cpp
+++ b/src/lib_json/json_value.cpp
@@ -17,9 +17,7 @@
 #include <sstream>
 #include <utility>
 
-#ifdef JSONCPP_HAS_STRING_VIEW
-#include <string_view>
-#endif
+
 
 // Provide implementation equivalent of std::snprintf for older _MSC compilers
 #if defined(_MSC_VER) && _MSC_VER < 1900