Interoperability between CMAKE_BUILD_TYPE and PSP_DEBUG

[timkpaine]

CMAKE_BUILD_TYPE is the standard, and is configured in DEBUG or RELEASE. We use PSP_DEBUG in the C++ code to trigger debug statements, and rely on it in cmake to build with debug symbols and without optimizations. This PR lets PSP_DEBUG be set from either the PSP_DEBUG environment variable, or from the CMAKE_BUILD_TYPE flag for cmake.

[texodus]

Thanks for the PR - this is a great improvement to the build process!