f2fs-tools: Fix cross-build problem caused by AC_CHECK_FILE abuse

f2fs-tools: Fix cross-build problem caused by AC_CHECK_FILE abuse

f2fs-tools fails to cross build from source, because its upstream
build system wrongly uses AC_CHECK_FILE. The purpose of the macro is
checking for files on the host system. However here it is used to
check whether a .git directory exists, which is a property of the
build place. A simple "test -d" should be used here. After doing so,
f2fs-tools cross builds successfully.

Addresses-Debian-Bug: #896909

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>