Fix CMake configuration for Windows build

There are two fixes in here:

1. Renaming the options to use the JANSSON_ prefix missed the place where the STATIC_CRT option was actually used
2. When building with STATIC_CRT, the previous CFLAGS are overridden completely, building the Debug configuration with Release flags (e.g. /O2, no /Zi).

I can't find anything in the CMake docs about how the  C_FLAGS variables are supposed to work, but repeating the previous value in the assignment works (it does *not* pass both /MD and /MT).