CMake 3.15 required, but 3.14 stated as minimum

[hedtke]

What version of OR-Tools and what language are you using?
Version: v9.0
Language: C++

Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)
Gurobi

What operating system (Linux, Windows, ...) and version?
Linux (Debian 10)

What did you do?
Steps to reproduce the behavior:

1. Install CMake 3.14 as this is set as minimum in the CMakeLists.txt
2. git clone https://github.com/google/or-tools -b v9.0
3. cd or-tools
4. cmake -S. -Bbuild -DCMAKE_BUILD_TYPE=Release -DBUILD_DEPS=ON -DUSE_SCIP=OFF

What did you expect to see
a running build

What did you see instead?

#6 188.1 CMake Error at cmake/dependencies/CMakeLists.txt:55 (list):
#6 188.1   list does not recognize sub-command POP_BACK

Many of the CMake files use list(POP_BACK ...) which does not exist in CMake 3.14.
The minimum version has to be set to 3.15

[Mizux]

Thanks for the feedback, I think we will move to CMake 3.18 since I need it to have python stuff working on a manylinux image

ref: https://cmake.org/cmake/help/latest/module/FindPython3.html
ref: https://gitlab.kitware.com/cmake/cmake/-/issues/20425

Pet project: https://github.com/Mizux/python-native