KeyError: 'b_vscrt' when building with clang.

[fryeb]

Describe the bug I get an error with --backend vs when the environment variable CC is set to clang. This same error does not occur with CC=cl or CC=clang-cl

To Reproduce I have been able to reproduce this bug with a handful of projects, including the tutorial project.

Expected behavior Having read the docs, I understand that clang is not a supported compiler under the visual studio backend. However this ought to be caught by an appropriate error message.

system parameters

• Is this a cross build or just a plain native build (for the same computer)? Native Build
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.) Windows 10
• what Python version are you using e.g. 3.8.0 3.8.2
• what meson --version 0.54.0
• what ninja --version if it's a Ninja build 1.9.0.git.kitware.dyndep-1.jobserver-1

[dcbaker]

This is a general problem with our visual studio backend, we assume that the compiler is going to be msvc-like (clang-cl, intel-cl). We'd have the same problem using GCC, which should work.