cpp_cc_git_submodule helper requires newer version of CMake than documented

The `BUILD` and `PACKAGE` keyword arguments to `cpp_cc_git_submodule` are supposed to work as flags even when they are not passed values. i.e.
```cmake
cpp_cc_git_submodule(foo)
cpp_cc_git_submodule(foo BUILD)
cpp_cc_git_submodule(foo BUILD bar)
```
are all supposed to do different things. This relies on the `_KEYWORDS_MISSING_VALUES` output of `cmake_parse_arguments`, which was only added in CMake v3.15. Without this feature IIUC we cannot distinguish between the first two cases. The coding conventions' README [states the minimum version is v3.10](https://github.com/BlueBrain/hpc-coding-conventions/blob/91c5d922a1b45943c6d2eca719088b547243f75c/cpp/README.md#cmake-project).

Either we should explicitly state that v3.15 is required, or `cpp_cc_git_submodule` should be modified for compatibility with older versions.