`add_compile_definitions` command not available in cmake 3.8.2

Describe the bug
You cannot build library with cmake in version 3.8.2.

To Reproduce
Make sure your cmake version is 3.8.2. Build the library.

Expected behavior
CMake fails to configure build since add_compile_definitions is not available in version 3.8.2.

Desktop (please complete the following information):

• OS: windows 10

[baylesj]

Good catch. We say we support version 3.8.0, but sounds like we don't.

[dota17]

In CMake 3.12 Release Notes， add_compile_definitions may be not added until CMake 3.12.

From PR #932 , add_definitions was changed to add_compile_definitions.
But in CMakeList.txt (root directory) , the oldestversion is still 3.8.0

set(JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION "3.8.0")

Maybe we should change the above message or revert some change of the cmake cleanup.

[cdunn2001]

I have found Cmake to be nearly impossible to maintain because different versions are so very different. I only bother with Meson, and I generally accept any PR submitted for Cmake.

If someone has a fix for this, great. If the solution is to specify a later version of Cmake, that's fine with me.

[hjmjohnson]

As this project is using C++11 features, cmake provides more robust support for C++11 features starting with version 3.12 and greater.

The new Cmake feature of "add_compiler_definitions" is certainly more appropriate, and more robust, but it a new feature that is only available in cmake 3.12 and greater. Ubuntu 18.04 envirionments come standard with cmake 3.10, so using the bleeding edge features of cmake is probably not a good idea for a project like jsoncpp. I'll have a look at fixing this later this week.