CMake by default initializes CMAKE_BUILD_TYPE as an empty string. Previously we defaulted it to Release, which sets particular compiler options. However, in some cases we do not want to set extra compiler options. For example, Linux packagers typically want to control exactly what compiler options are used, which we should respect and not try to override. Thus, the default build type (empty string) is ideal for letting builders control exactly what compiler options should be used.

Hence let the default build type be left undefined, with no additional compiler flags other than what was set in the CFLAGS environment.