+ Update build method on build system to usw BuildType enum.
+ Set CMake variable REZ_BUILD_TYPE based on value of BuildType enum.
+ Set CENTRAL CMake variable if in backwards compatibility mode and BuildType is central.
+ Set REZ_IN_REZ_RELEASE environment variable if in backwards compatibility mode and BuildType is central.
+ Update Doxygen macro to use new REZ_BUILD_TYPE CMake variable.