This isn't always needed, for example when using only the interpreter API,
and its dependency to native libraries on Linux (namely fontconfig) makes it difficult to cross-compile with
CMake.
While cargo is invoked correctly for the native binary build, the library search paths
coming from CMake will be target specific, not host specific.
So for now this provides a way of disabling the compiler.
Do not break building against slint using CMake from C++ when no
CMAKE_BUILD_TYPE was set when building Slint.
This just makes sure the build type is properly quoted, which makes sure
we keep a (empty) string token where CMake expects one.
Fixes: #1065
