Compile with latest supported C standard

Use latest C standard known to CMake. Remove CMAKE_C_STANDARD_REQUIRED
to fall back to earlier versions.

AFAIK, there's no way to set the minimum required version. But since we
support C99 and it's unlikely that CMake supports C89-only compilers,
this shouldn't be a problem.

Author: nwellnhof

CMakeLists.txt

@@ -4,8 +4,11 @@ project(cmark
   LANGUAGES C CXX
   VERSION 0.31.2)
 
-set(CMAKE_C_STANDARD 99)
-set(CMAKE_C_STANDARD_REQUIRED YES)
+if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.21)
+  set(CMAKE_C_STANDARD 23)
+else()
+  set(CMAKE_C_STANDARD 11)
+endif()
 set(CMAKE_C_EXTENSIONS NO)
 
 if(CMAKE_BUILD_TYPE STREQUAL Asan)