Compiler definitions not reloaded when switching only the Kit

When you switch only the Kit in the bottom-right menu, the compiler configuration isn't changed.

Steps to reproduce:

1. Have your project have build configuration in two or more kits
2. Confirm that your source code properly detects the compiler/kit (inspecting the code model suffices)
3. Switch kits but don't click any other Build config

Expected behaviour:

The build config is applied

Actual behaviour:

The previous build (in the non-selected Kit) is still active.

Workaround:

Select another build in the new Kit, then select the build you want.