Always use consistent CXX_STANDARD

Since CMake has subdirectory variable scope, unilaterally set the
CMAKE_CXX_STANDARD variable to use C++11. This covers cases with the
library being included externally, both in cases of only C++98 being
specified, as well as later versions being specified (since the
CXX_STANDARD itself isn't a library dependency, only the PUBLIC
target_compile_features on jsoncpp_lib). The previous direct check for
C++98 is handled by requiring C++11 on this library; should the
compiler being used not support C++11 then CMake will issue an error.

Author: mrjoel

CMakeLists.txt

@@ -37,24 +37,11 @@ foreach(pold "") # Currently Empty
     endif()
 endforeach()
 
-# ==== Define language standard configurations requiring at least c++11 standard
-if(CMAKE_CXX_STANDARD EQUAL "98")
-    message(FATAL_ERROR "CMAKE_CXX_STANDARD:STRING=98 is not supported.")
-endif()
-
-#####
-##  Set the default target properties
-if(NOT CMAKE_CXX_STANDARD)
-    set(CMAKE_CXX_STANDARD 11) # Supported values are ``11``, ``14``, and ``17``.
-endif()
-if(NOT CMAKE_CXX_STANDARD_REQUIRED)
-    set(CMAKE_CXX_STANDARD_REQUIRED ON)
-endif()
-if(NOT CMAKE_CXX_EXTENSIONS)
-    set(CMAKE_CXX_EXTENSIONS OFF)
-endif()
-
-# ====
+# Build the library with C++11 standard support, independent from other including
+# software which may use a different CXX_STANDARD or CMAKE_CXX_STANDARD.
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # Ensure that CMAKE_BUILD_TYPE has a value specified for single configuration generators.
 if(NOT DEFINED CMAKE_BUILD_TYPE AND NOT DEFINED CMAKE_CONFIGURATION_TYPES)