[tools] CMake 3.15 required

design/###-tools-cmake.md

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+
+# Proposal: Tools - CMake 3.15
+
+| **Status**        | **Proposed/Accepted/Deprecated** |
+|:------------------|:---------------------------------------------|
+| **RFC #**         | ####                                         |
+| **Submitted**     | YYYY-MM-DD                                   |
+| **Dependencies**  | RFC #, #                                     |
+
+---
+
+## Summary
+Declare CMake 3.15 as the lowest supported CMake version.
+
+
+## Motivation
+CMake is one of the main external tool we call from Conan, transparent integration and
+reliability should be a priority. It is important to declare a minimum supported
+version so we can run all the tests and be confident that the user will get the
+expected results and no regressions will be introduced.
+
+
+## Proposal
+CMake 3.15 was [released on July 2019](https://github.com/Kitware/CMake/releases/tag/v3.15.0). Significant additions are:
+
+ * (CMake 3.14) _Visual Studio 16 2019_ generator.
+ * Variable `CMAKE_PROJECT_INCLUDE` to add a file to be included after the `project()`
+   call. This is used by Conan toolchains.
+ * Variable `CMAKE_MSVC_RUNTIME_LIBRARY` to select the runtime library library used 
+   when targeting MSVC ABI.
+
+[Link to the full changelog](https://cmake.org/cmake/help/latest/release/3.15.html).
+
+
+## Alternative Approaches
+
+
+## Detailed Design
+
+
+## Open issues
+
+
+## Future Extensions