Do not override CMake's platform-dependent CFLAGS

[hattne]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

The need to build libarchive with /MD to use a multithreaded DLL runtime has been recognized before: the repository has 0009-CMake-Force-Multi-threaded-DLL-runtime.patch which adds /MD to the cl command line, but that patch is currently not applied. This PR addresses the problem by omitting CMAKE_C_FLAGS_RELEASE in the CMake invocation. CMake will then honor default flags for the platform (Modules\Platform\Windows-MSVC.cmake, in this case) in addition to CFLAGS from the environment.

This also means that NDEBUG will be defined for a release build, which, according to a comment in build_libarchive.dat will break the tests. However, the tests are disabled, and several of them fail, regardless of whether NDEBUG is defined or not. Defining NDEBUG seems inconsequential.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.