cmake: replace explicit checks for CMAKE_BUILD_TYPE

[dweindl]

In our CMakeLists.txt:

• Don't change CMAKE_BUILD_TYPE from within CMakeLists.txt. This should be passed to cmake at configure-time instead. This will also fix #2442.
• Don't check for CMAKE_BUILD_TYPE explicitly -- not compatible with multi-config generators
• Set MAP_IMPORTED_CONFIG_CONFIG

See also https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#build-configurations