ENH: Provide range for non-warned cmake versions

Allow configuring without cmake policy developer warnings
for a range of cmake versions.

This prevents the need to explicitly enumerate every new
policy for each new cmake version.

===

Moved setting of the CMAKE_CXX_STANDARD to before the project()
directive.

Author: hjmjohnson

CMakeLists.txt

@@ -1,6 +1,60 @@
 # vim: et ts=4 sts=4 sw=4 tw=0
 
-cmake_minimum_required(VERSION 3.1)
+# ==== Define cmake build policies that affect compilation and linkage default behaviors
+#
+# Set the JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION string to the newest cmake version
+# policies that provide successful builds. By setting JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION
+# to a value greater than the oldest policies, all policies between
+# JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION and CMAKE_VERSION (used for this build)
+# are set to their NEW behaivor, thereby suppressing policy warnings related to policies
+# between the JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION and CMAKE_VERSION.
+#
+# CMake versions greater than the JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION policies will
+# continue to generate policy warnings "CMake Warning (dev)...Policy CMP0XXX is not set:"
+#
+set(JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION "3.1.0")
+set(JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION "3.13.1")
+cmake_minimum_required(VERSION ${JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION} FATAL_ERROR)
+if("${CMAKE_VERSION}" VERSION_LESS_EQUAL "${JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION}")
+    #Set and use the newest available cmake policies that are validated to work
+    set(JSONCPP_CMAKE_POLICY_VERSION "${CMAKE_VERSION}")
+else()
+    set(JSONCPP_CMAKE_POLICY_VERSION "${JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION}")
+endif()
+cmake_policy(VERSION ${JSONCPP_CMAKE_POLICY_VERSION})
+#
+# Now enumerate specific policies newer than JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION
+# that may need to be individually set to NEW/OLD
+#
+foreach(pnew "") # Currently Empty
+    if(POLICY ${pnew})
+        cmake_policy(SET ${pnew} NEW)
+    endif()
+endforeach()
+foreach(pold "") # Currently Empty
+    if(POLICY ${pold})
+        cmake_policy(SET ${pold} OLD)
+    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()
+
+# ====
 
 # Ensures that CMAKE_BUILD_TYPE has a default value
 if(NOT DEFINED CMAKE_BUILD_TYPE)
@@ -73,12 +127,6 @@ if(MSVC)
     set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /W4 ")
 endif()
 
-# Require C++11 support, prefer ISO C++ over GNU variants,
-# as relying solely on ISO C++ is more portable.
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
-
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     # using regular Clang or AppleClang
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wconversion -Wshadow -Werror=conversion -Werror=sign-compare")