Suppress cmake policy CMP0048 warning when building with cmake 3.x

[simonrepp]

I am building with cmake 3.13.4 and getting the following warning during running cmake:

CMake Warning (dev) at CMakeLists.txt:3 (project):
  Policy CMP0048 is not set: project() command manages VERSION variables.
  Run "cmake --help-policy CMP0048" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  The following variable(s) would be set to empty:

    CMAKE_PROJECT_VERSION
    CMAKE_PROJECT_VERSION_MAJOR
    CMAKE_PROJECT_VERSION_MINOR
    CMAKE_PROJECT_VERSION_PATCH
This warning is for project developers.  Use -Wno-dev to suppress it.

According to my understanding and research this refers to a feature that is not used in this project (also the somewhat related custom OMMPFRITT_VERSION_MAJOR etc. variables as far as I can see don't get used currently). By setting the policy to NEW explicitly (as done in the PR) everyone who builds themselves is not getting the abovementioned and possibly confusing warning. Some empty variables (PROJECT_VERSION_... etc.) are set due to this change but this should have to my understanding no consequences and in any case will be the default behavior in the future.

[simonrepp]

For reference, I researched this (among other sources) here:

• CMake policy CMP0048 warning Tencent/rapidjson#1154
• https://cmake.org/cmake/help/latest/command/project.html

[pasbi]

I was annoyed by this warning for so long already, thanks for looking into this!
I don't really understand what's going on here but I guess you know what you do...