CMAKE_BUILD_TYPE=None should be allowed

[Matt Smith]

Libarchive cannot be compiled with the parameter CMAKE_BUILD_TYPE='None'.

I use bazel with the ninja generator, and would prefer to control all flags via the bazel toolchain rather than have cmake append it's own flags. Setting CMAKE_BUILD_TYPE='None' is a recommendation from ArchLinux.

Steps to reproduce the behavior: Compile latest libarchive code and compile it with cmake -DCMAKE_BUILD_TYPE='None'.