let CMakeToolchain not crash if build_type not defined

[memsharded]

Changelog: Fix: CMakeToolchain will not crash if build_type not defined.
Docs: Omit

@KerstinKeller

[lasote]

Missing a bit of context, there is an issue we can link? I assume there is something to discuss (look into label).

[memsharded]

Missing a bit of context, there is an issue we can link? I assume there is something to discuss (look into label).

Yes, this came from slack conversation with user (and Tribe member):

• They have multi-config packages, containing both Debug and Release
• They don't use CMakeDeps, because they use cmake_paths to get the config.cmake files from inside the packages
• The new CMakeToolchain was raising errors if they don't define the build_type setting.

Even if the tribe proposal said that cpp_info.configs is being removed and the plan is that users should migrate to single-config packages, there still will be some users in this situation. It seems that this PR can help them, but probably we want to carefully consider the implications.

[KerstinKeller]

The problem can also be fixed by removing the vs_runtime block for generation

tc.blocks.remove("vs_runtime")

For my usecase it is good enough, so I guess, for us, strictly speaking the PR is not neccessary.