Silence warnings building with CMake v3.31

[hjmallon]

• I've checked this Git style guide. [Yes]
• I've checked this CMake style guide. [Yes]
• My change will work with CMake 3.5 (minimum requirement for Hunter). [Yes]
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. [Yes]

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With CMake 3.31 you get warnings if you tell cmake to use a compat version <3.10. Change the version in a backwards compatible way to silence the warnings by upping the compat version to 3.12 but leaving the min version as 3.5

See #767

Also a gate change here cpp-pm/gate#17

On modern cmake this will enable policies CMP0066 through CMP0075 (https://cmake.org/cmake/help/latest/policy/CMP0066.html)