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

Commit 95f31e9a 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>
MinGW: Update tests to handle a native eol of crlf

Some of the tests were written with the assumption that the native eol would always be lf. After defining NATIVE_CRLF on MinGW, these tests began failing. This change will update the tests to also handle a native eol of crlf.

Signed-off-by: Brice Lambson <bricelam@live.com>