CMake: Enable configuration of CMAKE_BUILD_TYPE

[dg0yt]

msh3 used to hardcode the value of CMAKE_BUILD_TYPE to Release, in the middle of CMakeLists.txt. This is unexpected and a source of problems.

• In general, CMAKE_BUILD_TYPE is a variable which is meant to be controlled by the user. Also package managers like vcpkg make use of this as a general input variable.
• Setting a normal variable to a hard coded variable in the middle of the build script will hide the user input from this point. But users may easily overlook the trigger for behavioral inconsistencies they observe for different parts of the build.
• CMake encourages to use a different value than the default empty string.
  The natural way to implement defaults for user controlled variables is using cache variables.

This PR aligns msh3 with common practice: It sets a default early in CMakeLists.txt, as a cache variable subject to user control.

[nibanks]

Can you please provide more description as to why this is necessary?

[dg0yt]

Updated description.