config: don't rely on CMAKE_CONFIGURATION_TYPES

[mathstuf]

trafficstars

This can be set by a user by mistake. The global property is the trustworthy location for this information.

Also set the standard build types as options on the build type so it is easy to switch in ccmake or cmake-gui.

[drdanz]

Technically, this is correct, but GENERATOR_IS_MULTI_CONFIG is available only on CMake 3.9 or later, and this project still allows CMake 3.5

I'm in favour of porting this to CMake 3.12 (which is the CMake version we now require on the development branches), and accept this PR after we do that