Push the NATIVE_CRLF Makefile variable to C and added a test for native.

The previous patch correctly points out that the NATIVE_CRLF setting is
incorrectly set on Mingw git. However, the Makefile variable is not
propagated to the C preprocessor and results in no change. This patch
pushes the definition to the C code and adds a test to validate that
when core.eol as native is crlf, we actually normalize text files to this
line ending convention when core.autocrlf is false.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>