build: default to using RELWITHDEBINFO configuration
When using CMake, it appears to default to the debug configuration if CMAKE_BUILD_TYPE is not explicitly passed on the command line.
In the past, I had overridden it by doing: https://github.com/inolen/redream/commit/71d27486a252036e3e79e7f4f7fb2272a0cb0bab
However, when libsoundio was added as a dependency this broke for some reason and at the time I just removed it to work around it. It would be nice to get this back in, as the debug build isn't very playable (no fastmem, no texture cache invalidations), and many users accidentally build it their first time trying out redream.